Final answer:
The vehicle's speed while backing up should be controlled by the driver's foot on the brake pedal, ensuring safe and precise maneuvering.
Step-by-step explanation:
The vehicle's speed while backing should only be controlled by the driver's foot on the brake pedal. This is true because when backing up, it is critical to maintain a slow speed and be prepared to stop quickly if necessary. Using the brake pedal allows for fine control over the vehicle's speed and ensures that if the driver needs to stop suddenly, they can do so promptly and safely. The accelerator pedal can cause the vehicle to speed up, which is often not desired when backing up. The steering wheel and gear shift lever are used to control the direction of the vehicle and the transmission settings, respectively, but do not directly control the speed.