A tractor and a semi-trailer are being coupled. Although you have backed up, you are not yet beneath it. Which of the below should you hook up before backing under the semi-trailer?

A the emergency and service airlines.

explanation

You must lock the trailer brakes before backing beneath the trailer, which requires that you first confirm that the trailer brakes are operating. After testing the trailer brakes, connect the emergency and service airlines.
[Commercial Driver's License Manual]