What must the air pressure be in the system before you can operate the vehicle?

Answer: C — At least 100 psi

The correct answer is that the air pressure in the system must be at least 100 psi before you can operate the vehicle. This requirement is essential to ensure that the air brake system has sufficient pressure to function effectively. Air brakes operate by using compressed air, and the system must reach a certain pressure to generate the necessary force to engage the brakes properly. Operating a vehicle with insufficient air pressure can lead to brake failure, compromising safety on the road. The air brake system is designed to function best within specific pressure ranges, and 100 psi is the threshold that guarantees adequate pressure for reliable braking performance. Lower pressures may not provide enough force to engage the brakes fully, leading to a risk of accidents or loss of vehicle control.