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Question
Pull out the trailer air supply control to test:
Answers
A.  The parking brake.
B.  The trailer emergency brakes.
C.  The trailer service brakes.
D.  The landing gear.
Explanation
Be sure to test the trailer emergency brakes before beginning a trip. After ensuring that the trailer rolls freely, you can test the emergency brakes by pulling out the trailer air supply control, or placing it in the "emergency" position. Pull forward slightly with the tractor and make sure the trailer does not move.

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