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Question
The emergency air line:
Answers
A.  Is controlled by the foot brake.
B.  Is connected to the relay valves.
C.  Supplies air to the trailer air tanks.
D.  Prevents skids.
Explanation
The emergency air line (also known as the supply line) has two functions. The first is to supply air to the trailer air tanks on a combination vehicle, and the second is to control the emergency brakes.

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