Relatively small amounts of alcohol affect tolerance to hypoxia (lack of sufficient oxygen). This tolerance
Why this is the answer
Per TC AIM-AIR (Alcohol), even small amounts of alcohol reduce the body's tolerance to hypoxia, and that tolerance deteriorates further as altitude increases — the effects of alcohol and altitude are additive. It does not improve with altitude or stay constant.
Reference
TC AIM-AIR – Alcohol
Question reproduced verbatim from Transport Canada’s PSTAR Study and Reference Guide (TP 11919E). The answer key is Transport Canada’s; the explanation is ours.
A study aid — not a substitute for the official guide, the CARs, or your instructor.