Which of the following vehicles must not coast when going downhill?

Answer: B — Heavy-duty truck

Heavy-duty trucks must not coast when going downhill because doing so can significantly impair their ability to control speed and maneuver effectively. These vehicles are often equipped with air brakes that require a certain level of pressure to operate safely. Coasting can lead to a lack of engine braking, which is crucial for maintaining control on steep declines. In addition, heavy-duty trucks are larger and heavier than other types of vehicles, meaning that they need more distance and time to stop. By not coasting, drivers can engage the brakes more proactively, allowing for safer deceleration and reduced risk of brake overheating or failure. Choosing to maintain a controlled descent through proper gear selection helps prevent situations where the vehicle gains too much speed, which could lead to accidents or loss of control.