On a downgrade, the use of air brakes is considered a supplement to what?

The correct choice highlights that air brakes on a downgrade assist and enhance the braking effectiveness of the engine's braking system. When descending a hill or a steep grade, the vehicle's weight can significantly increase the risk of losing control due to acceleration from gravity. The engine's braking effect, often achieved by downshifting to lower gears, helps to slow down the vehicle naturally.

Air brakes are used in conjunction with this engine braking to provide additional stopping power, ensuring that the vehicle remains under control and does not gain excessive speed. This combination is especially vital for larger vehicles, such as trucks, which may have more momentum when descending hills. By effectively utilizing both the engine's braking capacity and the air brakes, drivers can maintain safer speeds and enhance overall driving safety on downgrades.