On a long, steep downgrade, how do brakes function?

Answer: B — As a supplement to engine braking

Brakes function as a supplement to engine braking on long, steep downgrades because engine braking is crucial for controlling vehicle speed on such slopes. When descending steep grades, a driver can downshift to a lower gear, allowing the engine to help slow down the vehicle through compression and friction. This engine braking effect works in tandem with the brakes to prevent overheating and brake fade, which can occur from continuous use of the braking system alone. Therefore, while the brakes are indeed utilized, their primary role on steep downgrades is to assist the engine braking, helping maintain a safe speed and ensuring the brakes do not wear out prematurely. This understanding is vital for maintaining vehicle control and safety on such terrains.