Explain why the driver of a car has to continuously apply pressure to the gas pedal in order to keep the car cruising at a steady speed, even on a flat road.

The gas pedal opens the throttle. This supplies air to the engine and fuel is metered into the air stream to provide a combustible mixture to the engine. If the throttle is close, I.e., you lift you foot, this process stops and the engine slows to an idle setting where enough fuel is provided to keep the engine turning over but not to move the car at any significant speed. When your cruising at a steady speed, even on a flat road, the engine turns over faster and requires enough fuel so that air resistance are overcome as well as internal friction losses in the engine.