Question

【题目】— What were you doing I knocked at the door?
— I was sleeping.
A.unless
B.once
C.when
D.while

Answer 1

【答案】C
【解析】考查连词辨析。句意：-我敲开门时，你在做什么？-我在睡觉。A. unless 如果不，除非；B. once一旦……，引出时间状语从句；C. when当……时；D. while当……时，后跟延续性动词。本题表示“当……时”，knock是非延续性动词，故选B。