Last night, before the exam, I went over the lessons I couldn’t keep my eyes open to stay awake any longer.
A. before
B. until
C. unless
D. when
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A. before
B. until
C. unless
D. when
解析：选B。考查状语从句。句意: 昨天晚上, 考试前, 我复习功课直到我再也难以支撑的时候。until直到……的时候; before在……之前; unless除非; when当……的时候。