What is true about downshifting for a long downhill grade?

Answer: C — It allows engine compression and friction to help slow the vehicle.

When descending a long downhill grade, downshifting is crucial for vehicle control, and the correct choice highlights the advantages of this technique. Downshifting allows the engine to utilize compression and friction to help slow the vehicle down. This is particularly important in maintaining control during a descent. By engaging a lower gear, the engine can create additional resistance that counteracts the force of gravity pulling the vehicle down the hill. This engine braking effect reduces reliance on the brake system, which can overheat and become less effective if used excessively. The result is that the driver can maintain a safer, steadier speed while controlling the vehicle more effectively, minimizing the risk of brake failure. Utilizing engine compression is especially vital for larger vehicles, like trucks, where the weight can be significant; relying solely on brakes can lead to dangerous situations.