Before backing under the trailer, you should check that:

The trailer brakes are locked.. Section 6.4: Trailer brakes must be locked.

Before backing under the trailer, you should check that:

ANSWER BREAKDOWN
  • A. Correct. Trailer brakes must be locked.
  • B. Incorrect. "The trailer lights are on." is not correct. Trailer brakes must be locked.
  • C. Incorrect. "The doors are open." is not correct. Trailer brakes must be locked.
  • D. Incorrect. "The cargo is unloaded." is not correct. Trailer brakes must be locked.