You should never leave the vehicle unattended without:
Correct. Set the parking brake. On grades or where unintended movement could occur, chock the wheels in addition.
CDL Practice Question #139
Category: Air Brakes
Why this question matters
This question is one of 71 in our Air Brakes 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.