Transportation, Warehousing, and Material Handling

Introduction

Reverse logistics concerns the movement, storage, inspection, disposition, and recovery of products, components, packaging, and materials after their sale, use, or return. Transportation, warehousing, and material handling are not merely supporting activities in this system. They determine whether returned items can be recovered economically, whether hazardous materials are controlled safely, whether product identity and condition remain traceable, and whether recovery is faster and less environmentally damaging than disposal.

Forward logistics generally moves standardized products from a small number of suppliers toward many customers. Reverse flows are more dispersed, uncertain, heterogeneous, and condition-dependent. Returned products may arrive in small quantities from thousands of locations; packaging may be damaged; documentation may be incomplete; and the ultimate disposition may not be known at collection. Consequently, reverse logistics facilities and transportation networks must be designed for uncertainty and decision flexibility rather than only for high-volume flow efficiency.

This chapter examines how managers configure reverse transportation, backhauling, consolidation, storage, packaging, hazardous-material handling, and cross-docking while balancing cost, speed, safety, traceability, and environmental impact.

Learning Objectives

By the end of this chapter, students should be able to:

1. Distinguish reverse transportation from forward transportation and identify the operational consequences of uncertain returns.
2. Evaluate backhauling, consolidation, milk runs, and dedicated reverse transport.
3. Design storage and material-handling processes for inspection, repair, reuse, recycling, and disposal.
4. Select packaging and handling methods that protect products, workers, and the environment.
5. Explain the control requirements for hazardous and regulated materials.
6. Assess when cross-docking is appropriate in a reverse logistics network.
7. Use cost, service, safety, traceability, and environmental criteria in configuration decisions.

1. The Operating Logic of Reverse Flows

A reverse logistics flow can be represented as:

Customer or user → collection point → consolidation facility → inspection and disposition → recovery, resale, remanufacture, recycling, or disposal.

The sequence is not always linear. Products may move backward to a retailer, directly to a repair center, or to a specialized recycler. Some items are returned to stock; others are routed immediately to a secondary market. The central operational decision is the disposition decision: what should happen to an item given its identity, condition, ownership, legal status, residual value, and recovery cost?

Reverse flows differ from forward flows in five important respects:

These differences create a trade-off between consolidation and responsiveness. Consolidation lowers transportation cost per unit, but waiting for a full load increases cycle time and may reduce recovery value. Rapid movement protects value and customer service but can produce low vehicle utilization.

A useful total-cost structure is:

Here, each term is a cost over a defined planning period. C_transport includes line-haul, collection, fuel, accessorial, and carrier charges; C_handling includes loading, unloading, sorting, and internal movement; C_storage includes space, utilities, insurance, and inventory carrying cost; C_processing includes inspection, testing, repair, and repackaging; C_disposal includes treatment and disposal fees; C_risk represents expected loss from damage, noncompliance, theft, or injury; and C_environment represents measured or monetized environmental impacts when the organization includes them in its decision model. The equation is an accounting framework, not a claim that every cost can be measured with equal precision.

2. Reverse Transportation Design

2.1 Collection structures

Common collection structures include:

• Drop-off collection: Customers bring products to stores, depots, parcel locations, or municipal sites. It has low collection cost for the firm but depends on customer participation.
• Pickup collection: A carrier collects products from customers or service locations. It supports bulky or high-value items but is costly when stops are dispersed.
• Retail take-back: Retail stores accept products associated with their sales or regulatory obligations. It benefits from existing facilities and customer traffic.
• Service-network collection: Technicians collect failed parts, cores, batteries, or equipment during scheduled service visits.
• Mobile or scheduled collection: A vehicle visits multiple locations according to a planned route, often for reusable packaging or industrial scrap.

Collection design should consider product density, return frequency, value, hazard classification, customer convenience, and the quality of information captured at the point of return.

2.2 Dedicated transport, backhauling, and shared capacity

A dedicated reverse route is appropriate when returns are predictable, time-sensitive, hazardous, or incompatible with other cargo. Dedicated movement offers control and traceability but may generate empty mileage.

Backhauling uses a vehicle returning from a forward delivery route to collect reverse material. It can reduce empty running and exploit existing driver, vehicle, and scheduling capacity. However, it is effective only when pickup locations, vehicle capacity, loading time, product compatibility, and legal requirements align. A truck carrying food, pharmaceuticals, chemicals, or clean-room equipment may not be suitable for mixed returns.

A simple empty-mileage indicator is:

E is the percentage of vehicle distance traveled empty, D_empty is empty distance, and D_total is total distance. Both distances must use the same unit and planning horizon. The measure describes transport utilization but does not by itself prove that a route is efficient; a highly utilized route may create excessive waiting, handling, or risk.

Shared transportation combines reverse loads from multiple firms or product streams. It can improve utilization but requires compatible packaging, clear liability, standardized data, and segregation where contamination or regulatory risk exists.

2.3 Consolidation and milk runs

Consolidation combines small return quantities into larger movements to a regional processing or recovery facility. Its benefits include lower cost per unit, fewer vehicle movements, and improved carrier utilization. Its disadvantages include additional handling, delay, possible damage, and the risk that low-value items accumulate faster than they can be processed.

A milk run is a scheduled route that visits several collection points and returns to a consolidation hub. A route decision should balance distance, vehicle capacity, stop time, pickup windows, material compatibility, and service requirements. In practice, managers often use vehicle-routing software, but the software must receive reliable data on return volume, pallet dimensions, hazardous classifications, and loading constraints.

The consolidation decision can be expressed conceptually as:

The inequality applies to comparable service, safety, and time conditions. C_consolidated is the cost of consolidated movement, C_delay is the economic effect of waiting, C_extra_handling is additional handling cost and expected damage, and C_direct is the cost of direct shipment. The delay term may include loss of resale value or contractual penalties.

2.4 Transportation documentation and traceability

Reverse transport requires a chain of custody. Minimum data commonly include return authorization, product identifier, serial or batch number, quantity, origin, destination, condition, reason for return, carrier, timestamps, and hazardous-material classification where applicable. Barcodes, radio-frequency identification, mobile scanning, and electronic proof of receipt can reduce data loss, but technology cannot correct poor process discipline.

A traceability system should answer four questions:

1. What item or material was received?
2. Where did it come from and who had custody?
3. What handling, inspection, or transformation occurred?
4. Where was it finally transferred, recovered, or disposed of?

3. Warehousing for Uncertain and Heterogeneous Returns

3.1 The reverse logistics facility

A reverse facility is more than a storage building. It is a decision and transformation center. A typical process includes receiving, identification, quarantine, inspection, testing, grading, disposition, repair or disassembly, repackaging, storage, and outbound release.

A practical layout separates incompatible activities and minimizes unnecessary travel:

1. Receiving and documentation verification.
2. Quarantine for unidentified, damaged, contaminated, or potentially hazardous items.
3. Inspection and testing.
4. Grading and disposition coding.
5. Separate zones for resale, repair, parts harvesting, recycling, and disposal.
6. Secure storage for high-value or sensitive products.
7. Outbound staging.

The facility should be designed around process flow rather than simply available floor area. Returned products often need more touches per unit than forward products, so a layout that looks efficient for pallet storage may be inefficient for inspection and sorting.

3.2 Storage policies

Reverse inventory is usually highly variable in value and condition. Appropriate policies include:

• Quarantine storage for items awaiting identification, safety review, or ownership resolution.
• Condition-based storage separating unopened, tested, repairable, cannibalizable, recyclable, and nonrecoverable items.
• Time-sensitive storage for products whose value declines rapidly, such as seasonal goods or electronic devices.
• Secure storage for confidential, high-value, controlled, or counterfeit-sensitive products.
• Environmental-control storage for temperature-, humidity-, or contamination-sensitive materials.

The common error is to treat all returned inventory as equivalent. A returned laptop, a used medical device, a damaged lithium-ion battery, and a cardboard carton require different controls, even if each occupies one storage location.

Inventory carrying cost can be approximated as:

I_avg is average inventory quantity, v is value per unit, i is the carrying-rate fraction per unit time, and T is the time period. Units must be consistent: if i is annual, T must be in years. The approximation assumes average inventory and a stable carrying rate. It excludes nonlinear risks such as obsolescence unless those are added separately.

For reverse inventory, the effective carrying rate may be high because of obsolescence, theft, deterioration, and declining resale value. First-in, first-out is not always sufficient; managers may need first-expiring, first-out, priority by value decay, or priority by legal deadline.

3.3 Capacity and service trade-offs

Capacity should be planned for both average flow and variability. If a facility is designed only for average returns, peak periods may cause unsafe floor storage, misidentification, and delayed disposition. Excess capacity, however, increases fixed cost. Flexible labor, temporary staging, modular shelving, and appointment scheduling can help absorb peaks.

A useful performance measure is dock-to-disposition time:

T_DD is the elapsed time between recorded receipt and recorded disposition. It should be measured using synchronized timestamps and a defined treatment of exceptions such as missing documents. It is not the same as total customer return time, which also includes customer initiation and transport.

4. Packaging and Material Handling

4.1 Packaging objectives

Reverse packaging must protect the product, the handler, other cargo, and the environment. It should also support identification and efficient handling. Key design questions are:

• Can the original packaging be reused safely?
• Does the package prevent movement, leakage, puncture, or electrostatic damage?
• Is it compatible with pallets, totes, conveyors, and automated equipment?
• Can it be opened for inspection without destroying useful packaging?
• Can materials be separated and recycled?
• Does it contain required labels, warnings, and handling instructions?

Reusable packaging is attractive where return density and cycle frequency justify cleaning, inspection, tracking, and reverse transport. A single-use package may have lower operational complexity but higher material consumption and waste.

4.2 Unit loads and handling equipment

Pallets, totes, roll cages, bins, carts, conveyors, forklifts, automated storage systems, and guided vehicles are selected according to load dimensions, mass, fragility, contamination risk, throughput, and facility layout. Reverse operations often require flexible equipment because load profiles change and many items are not palletized.

Material-handling principles include:

• Minimize touches and travel distance.
• Keep heavy items within safe lifting limits and stable load zones.
• Use mechanical assistance for repetitive or awkward handling.
• Separate pedestrian and vehicle traffic.
• Inspect pallets, racks, slings, and containers.
• Standardize locations and labels.
• Prevent mixing of identified and unidentified material.

Ergonomic design is both an ethical obligation and a productivity strategy. Injuries, fatigue, and errors increase when workers repeatedly lift irregular returns, open damaged packages, or work in congested areas.

4.3 Packaging decisions and environmental impact

Environmental evaluation should include the entire activity rather than only package weight. A reusable container may be beneficial when the number of cycles and return distance offset cleaning and transport impacts. A conceptual life-cycle comparison is:

I_reuse is impact per use, m is the environmental impact of manufacturing the reusable package, e_manufacture should not be multiplied by mass unless the impact coefficient is defined per unit mass, n is the number of completed cycles, e_cleaning is impact per cleaning cycle, and e_return is the impact of returning the empty package. More rigorously, manufacturing impact should be represented as a total term E_manufacture, giving:

The second form is preferable because it avoids ambiguous units. Comparison with a disposable package requires equivalent protection and service. Environmental claims should identify system boundaries and data sources.

5. Hazardous and Regulated Materials

Hazardous reverse flows include batteries, aerosols, solvents, fuels, medical waste, chemicals, contaminated equipment, and products containing regulated substances. Reverse handling is especially difficult because the material may be damaged, incorrectly labeled, partially discharged, leaking, or mixed with ordinary returns.

A robust control system includes:

1. Identification at the point of return.
2. Classification according to applicable law and transport mode.
3. Inspection for leakage, swelling, damage, contamination, or incompatible contents.
4. Segregation by hazard class and compatibility.
5. Approved packaging, labels, marks, and documentation.
6. Trained and authorized personnel.
7. Emergency response equipment and procedures.
8. Secure storage with access control and inspection records.
9. Approved carriers and treatment or disposal providers.
10. Incident reporting and corrective action.

Regulatory requirements vary by jurisdiction and mode, including road, rail, air, and sea. Organizations must consult current national and international rules rather than rely on generic labels or historical practice. Examples of relevant frameworks may include dangerous-goods transport rules, occupational safety requirements, waste legislation, producer-responsibility obligations, and product-specific battery or medical-device requirements.

Hazard communication is a governance issue as well as an operational issue. Deliberately misclassifying a return to avoid a surcharge can expose workers, carriers, communities, and the organization to severe harm. Procurement contracts should specify who owns classification, packaging, documentation, insurance, and incident responsibility.

6. Cross-Docking in Reverse Logistics

Cross-docking transfers goods from inbound receiving to outbound staging with little or no storage. In reverse logistics, it is appropriate when the destination and disposition are already known, the product requires minimal inspection, and condition risk is low. Examples include unopened products returned to a supplier, standardized reusable packaging, and pre-authorized warranty exchanges.

Cross-docking is less suitable for mixed, unidentified, damaged, or hazardous returns requiring testing. Such items need quarantine and controlled inspection before release.

A reverse cross-dock requires:

• advance shipment and return data;
• clear disposition rules;
• dedicated staging lanes;
• rapid identification and scanning;
• compatible inbound and outbound schedules;
• exception handling for discrepancies;
• physical segregation of incompatible products.

The attraction is reduced storage and faster cycle time. The risk is that speed may bypass inspection, creating counterfeit, safety, warranty, or compliance exposure. Therefore, cross-docking should be based on risk classification rather than on a blanket target of zero storage.

7. Integrated Configuration Framework

A graduate-level configuration decision should use multiple objectives. A practical scorecard is shown below.

Managers should avoid optimizing one measure in isolation. For example, a low transport cost achieved by waiting months for consolidation may destroy resale value. A high cross-dock rate may reduce storage but increase safety and warranty risk. A reusable package may reduce waste while increasing empty-return miles.

A weighted scoring model can support screening decisions:

S is the overall score, w_j is the weight assigned to criterion j, and r_j is the normalized rating of an option on that criterion. Weights must be agreed transparently, ratings should use defined scales, and the model should be tested with sensitivity analysis. A score does not replace legal compliance: an illegal option cannot be justified by a high total score.

8. Technology, Data, and AI

Warehouse-management systems can assign locations, manage quarantine, and record disposition. Transportation-management systems can plan collection routes, consolidate loads, and document carrier performance. Barcodes and RFID improve item identity; sensors can monitor temperature, shock, humidity, or battery conditions; and digital records can improve chain of custody.

Artificial intelligence can assist with return-volume forecasting, image-based grading, route optimization, anomaly detection, and disposition recommendations. Its use requires caution. Training data may reflect historical bias, damaged products may be misclassified, and an algorithm may recommend a financially attractive disposition that conflicts with safety or environmental obligations. Human review is essential for hazardous, high-value, regulated, or ambiguous cases.

Good governance includes data ownership, access controls, audit trails, model validation, explainability, cybersecurity, and retention rules. A digital system is valuable only when workers scan consistently and master data remain accurate.

9. Illustrative Case: Electronics Manufacturer Take-Back Network

Background

An illustrative electronics manufacturer receives smartphones and tablets through retail stores, parcel returns, service centers, and corporate customers. Returns vary from unopened products to cracked screens, swollen batteries, counterfeit items, and devices containing customer data.

Problem

The company used a single regional warehouse. All returns were shipped weekly in mixed loads, placed in common storage, and inspected in arrival order. This produced long delays, battery safety incidents, lost serial-number data, and unnecessary transport of low-value devices.

Analysis

The company classified returns into four risk and value groups:

1. unopened and authorized for rapid resale;
2. functional products requiring data erasure and testing;
3. repairable products and parts candidates;
4. damaged or hazardous products, especially batteries.

The analysis showed that group 1 could use controlled cross-docking, group 2 required secure processing, group 3 benefited from regional consolidation, and group 4 required compliant hazardous-material packaging and specialized treatment. Backhauling from retail stores was feasible for groups 1–3, but battery shipments required approved packaging and carrier procedures.

Approach and solution

The manufacturer introduced return authorization and serial scanning at collection. Retail deliveries were combined with scheduled pickups where capacity and compatibility allowed. A quarantine zone was established for unidentified or damaged products. Cross-docking was limited to unopened, verified items. Secure cages were used for data-bearing devices, and a separate battery area included inspection, segregation, and emergency procedures. Disposition codes triggered different storage, transport, and processing routes.

A weighted decision model gave cost a 30% weight, speed 20%, safety 25%, traceability 15%, and environmental impact 10%. The company tested the result under different return peaks rather than relying on one average month.

Outcome

The redesigned process reduced unnecessary touches, improved serial-level traceability, accelerated resale of unopened products, and separated hazardous batteries from ordinary returns. The case also demonstrated that transportation savings alone were not the objective: the greatest value came from matching transport and facility controls to product condition and risk.

Learning points

• Reverse networks should segment flows before choosing transport modes.
• Cross-docking is a disposition-control decision, not simply a warehouse-speed initiative.
• Backhauling is valuable only when capacity, compatibility, timing, and compliance align.
• Safety and data protection must be designed into the process rather than added after incidents.
• Environmental performance should include packaging, transport, processing, and avoided disposal.

10. Challenges and Limitations

Reverse logistics decisions are constrained by uncertain volumes, incomplete data, fluctuating recovery prices, changing regulations, and conflicting incentives among manufacturers, retailers, carriers, recyclers, and customers. A retailer may want rapid removal of returns, while a manufacturer wants inspection before accepting ownership. A carrier may optimize route miles while the recovery operation values product freshness.

Quantitative models also have limitations. Cost estimates may omit reputational damage, injury risk, or the value of customer trust. Environmental estimates depend on system boundaries and data quality. Weighted scores can conceal disagreement behind a single number. Therefore, managers should use scenario analysis, sensitivity testing, pilot implementations, audits, and post-implementation measurement.

Best-Practice Checklist

1. Segment returns by condition, value, hazard, urgency, and legal status.
2. Capture identity and reason-for-return data at the earliest practical point.
3. Use backhauling only after checking capacity, compatibility, timing, and compliance.
4. Consolidate when transport savings exceed delay, handling, damage, and value-loss costs.
5. Establish quarantine and clearly marked disposition zones.
6. Design packaging for protection, handling, traceability, and end-of-life recovery.
7. Maintain current hazardous-material procedures and trained personnel.
8. Apply cross-docking selectively to verified, low-risk, pre-directed flows.
9. Measure cost, speed, safety, traceability, recovery value, and environmental impact together.
10. Audit data quality and use human oversight for high-risk automated decisions.

Chapter Summary

Reverse transportation, warehousing, and material handling determine whether returned products retain value and whether recovery occurs safely and efficiently. Reverse flows are uncertain, dispersed, condition-sensitive, and information-intensive. Backhauling and consolidation can reduce empty mileage and unit transport cost, but waiting and extra handling may destroy recovery value. Warehouses should therefore provide receiving, quarantine, inspection, grading, secure storage, repair, recycling, and controlled outbound staging rather than treating returns as ordinary inventory.

Packaging and material handling must protect products, people, cargo, and the environment. Hazardous materials require classification, segregation, compliant packaging, trained personnel, documentation, and emergency controls. Cross-docking can accelerate verified, pre-directed returns but is inappropriate for unidentified, damaged, or high-risk materials. Effective configuration uses total-cost analysis, traceability, risk segmentation, technology governance, and a balanced scorecard covering cost, speed, safety, environmental impact, flexibility, and recovery value.

Key Terms

Reverse logistics: Management of product, packaging, and material flows from use or customer location back to recovery or appropriate disposition.

Backhauling: Using a vehicle’s return trip after forward delivery to collect reverse materials.

Consolidation: Combining multiple small shipments into a larger movement.

Milk run: A scheduled route serving several collection or delivery points.

Quarantine: Controlled isolation of material awaiting identification, inspection, or safety review.

Disposition: The decision about resale, repair, reuse, recycling, return, or disposal.

Cross-docking: Moving inbound goods to outbound staging with little or no storage.

Chain of custody: Documented record of possession and handling from origin to final destination.

Unit load: Grouped material handled as one load, such as a pallet, tote, or roll cage.

Material handling: Movement, protection, storage, and control of materials within operational facilities.

Hazardous material: A substance or article presenting regulated risks to health, safety, property, or the environment.

Traceability: Ability to identify and reconstruct an item’s origin, location, handling, and disposition.

Recovery value: Economic or functional value obtainable through reuse, repair, resale, remanufacture, or recycling.

Dock-to-disposition time: Elapsed time from recorded receipt to recorded disposition.

Empty mileage: Vehicle distance traveled without a productive load.

Return authorization: Formal approval and information record supporting a return.

Condition grading: Categorization of a returned item according to physical, functional, and commercial condition.

Reusable packaging: Packaging designed for multiple service cycles.

Disposition code: Standard data classification that directs subsequent handling.

Weighted scoring model: Multi-criteria method that combines criterion ratings using assigned weights.

Review Questions

Short Answer Questions

1. Why are reverse logistics flows generally more uncertain than forward logistics flows?
2. Define backhauling and identify two conditions required for it to be effective.
3. What is the purpose of a quarantine area in a reverse logistics facility?
4. Why should cross-docking be restricted for some returned products?
5. Identify four types of information needed for reverse-flow traceability.

Long Answer Questions

1. Compare dedicated reverse transportation, backhauling, and consolidation. Explain the circumstances under which each is appropriate.
2. Design a warehouse process for mixed consumer-electronics returns, including receiving, quarantine, inspection, storage, disposition, and outbound movement.
3. Explain how packaging and material-handling decisions affect cost, worker safety, product recovery, and environmental performance.
4. Discuss the governance requirements for transporting and storing hazardous reverse materials.
5. Critically evaluate the use of technology and artificial intelligence in reverse logistics operations.

Analytical / Application Questions

1. A vehicle travels 1,200 km in a planning period, of which 300 km are empty. Calculate the empty-mileage percentage and explain one operational action that could reduce it.
2. A company is deciding between direct shipment and consolidation. Consolidation saves transport cost but adds waiting time and handling. Construct a decision analysis using the total-cost framework and identify the data required before selecting an option.
3. A retailer receives unopened products, repairable products, and damaged lithium-ion batteries. Propose a segmented transportation and warehouse configuration, explaining how cost, speed, safety, traceability, and environmental impact are balanced.

Multiple Choice Questions with Answers

Question 1

Which characteristic most distinguishes reverse logistics from forward logistics?

A. Returns are always transported by air

B. Return quantities and product conditions are often uncertain

C. Reverse flows never require warehousing

D. All returned products have identical value

Correct Answer: B. Return quantities and product conditions are often uncertain

Question 2

A truck travels 1,200 km, including 300 km without a productive load. What is its empty-mileage percentage?

A. 20%

B. 25%

C. 30%

D. 40%

Correct Answer: B. 25%

Question 3

Which situation is most suitable for backhauling?

A. A return load is incompatible with the forward cargo and requires special legal controls

B. A vehicle has compatible capacity on its return route and scheduled pickup locations

C. The return has no identification or packaging

D. The vehicle is already over its legal weight limit

Correct Answer: B. A vehicle has compatible capacity on its return route and scheduled pickup locations

Question 4

What is the primary purpose of quarantine storage?

A. To accelerate disposal without inspection

B. To isolate material awaiting identification, inspection, or safety review

C. To mix all returns into one inventory category

D. To replace hazardous-material documentation

Correct Answer: B. To isolate material awaiting identification, inspection, or safety review

Question 5

Which return is the strongest candidate for controlled cross-docking?

A. An unidentified leaking chemical container

B. A damaged battery awaiting classification

C. An unopened, verified product with a predetermined destination

D. A mixed pallet requiring extensive testing

Correct Answer: C. An unopened, verified product with a predetermined destination

Question 6

In the total-cost framework, which item represents the economic effect of waiting for consolidation?

A. C_delay

B. C_handling

C. C_disposal

D. C_transport

Correct Answer: A. C_delay

Question 7

Which practice best supports reverse-flow traceability?

A. Recording only the carrier’s name

B. Scanning product identifiers and recording custody and disposition events

C. Removing serial numbers before inspection

D. Storing identified and unidentified items together

Correct Answer: B. Scanning product identifiers and recording custody and disposition events

Question 8

Why may first-in, first-out be insufficient for reverse inventory?

A. Returned items never have value

B. All reverse inventory is hazardous

C. Value, expiration, and condition can vary substantially among returns

D. FIFO applies only to transportation

Correct Answer: C. Value, expiration, and condition can vary substantially among returns

Question 9

Which control is essential for hazardous reverse materials?

A. Ignoring damaged packaging until final disposal

B. Mixing incompatible materials to maximize vehicle utilization

C. Classification, segregation, compliant packaging, and trained personnel

D. Using ordinary labels to reduce handling time

Correct Answer: C. Classification, segregation, compliant packaging, and trained personnel

Question 10

In a weighted scoring model, what condition is normally imposed on criterion weights?

A. Every weight must equal zero

B. The weights must sum to one

C. The highest-cost criterion must receive weight zero

D. All ratings must be measured in kilograms

Correct Answer: B. The weights must sum to one