Final answer:
The first step when starting a tractor-trailer engine is to apply the parking brake for safety reasons, ensuring the vehicle remains stationary.
Step-by-step explanation:
The correct answer is option B. Apply the parking brake. This is an essential safety step to ensure that the vehicle does not move unexpectedly when the engine starts. When starting a tractor-trailer, or any large vehicle, it's crucial to first secure the vehicle by applying the parking brake.
After securing the vehicle, you can proceed with other steps such as depressing the clutch or turning the ignition key. Safety is a top priority when operating heavy machinery, and beginning with the parking brake helps to maintain control of the vehicle during the starting process.
The correct answer is option C. Turn the ignition switch or key on.
When starting a tractor-trailer engine, the very first step is to turn the ignition switch or key on. This allows the electrical systems in the vehicle to activate and prepare for engine ignition.
Once the ignition is turned on, the driver can then proceed to depress the clutch, apply the parking brake, and finally start the engine using the appropriate starting procedure for their specific vehicle.