Before starting down a hill, driver should set the vehicle in the proper gear ___________________.

A so you only have to apply the brake just hard enough to feel a definite slowdown.

explanation

The use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following are the proper braking techniques: Apply the brakes just hard enough to feel a definite slowdown. [Commercial Driver License Manual]