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Question
What does the trailer air supply control look like?
Answers
A.  It is a square blue knob.
B.  It is a triangular yellow knob.
C.  It is an eight-sided red knob.
D.  It is a circular orange knob.
Explanation
On newer vehicles, the trailer air supply control is an eight-sided red knob. It's used to control the tractor protection valve.

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