24. After connecting the air lines, but before backing under the trailer you should:

Combination Vehiclesmedium

AMake sure that the trailer brakes are off.

BSupply air to the trailer system, then pull out the air supply knob.

CWalk around the rig to make sure it is clear.

Detailed Explanation

The correct answer is "Supply air to the trailer system, then pull out the air supply knob.". This is based on federal CDL regulations and safety standards.

Correct Answer: Supply air to the trailer system, then pull out the air supply knob.

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24. After connecting the air lines, but before backing under the trailer you should:

Detailed Explanation

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