Which statement about what causes vehicle fires is true?

Answer: A — Poor trailer ventilation can cause flammable cargo to catch on fire.

Fires involving vehicles carrying cargo are often driven by ignition of flammable vapors, not just heat from mechanical parts. When a trailer isn’t well ventilated, vapors from flammable cargo can accumulate inside. If heat builds up or a spark is present, those vapors can ignite, making poor trailer ventilation a common and preventable fire risk. That’s why this statement is the best: it directly links the trailer environment to the mechanism that most frequently starts such fires. While overheating brakes can cause a fire in some scenarios, and other options are less likely sources of ignition under normal conditions, the risk from vapor buildup in a poorly ventilated trailer with flammable cargo best explains how these fires start.