During a pre-trip on doubles, the trailer-supply line:
Correct. The supply line carries air from tractor through every trailer; verify open valves at every coupled connection and a closed valve at the rear of the rear-most trailer.
CDL Practice Question #362
Category: Doubles/Triples
Why this question matters
This question is one of 50 in our Doubles/Triples bank. The reasoning above comes directly from the AAMVA CDL Manual, the source document used by every U.S. state DMV. Memorizing the underlying rule — not just the letter answer — is what separates passing on the first attempt from a return trip and another fee.