Last night before the exam I went over the lessonsI couldn’t keep my eyes open to stay a

问题补充：

Last night, before the exam, I went over the lessonsI couldn’t keep my eyes open to stay awake any longer．

A. before

B. until

C. unless

D. when

答案：

A. before

(答案→)B. until

C. unless

D. when

解析：考查状语从句。语境表示“直到我再也难以支撑的时候”，用until引导时间状语从句表示“直到……的时候”。