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You are coupling a tractor to a semi-trailer. You have backed up but are not yet under it. Before backing under the semi-trailer, you should hook up

A the emergency and service air lines.
Explaination
Before backing under the trailer, you must lock the trailer brakes – which means that you first have to make sure that the trailer brakes are working. Connect the emergency and service air lines and test the trailer brakes.
B the electrical service cable.
C nothing. Back up and lock the fifth wheel.