Question

【题目】They were smoking ______ the teacher appeared in front of them.
A.where
B.while
C.what
D.when

Answer 1

【答案】D
【解析】句意：他们正在抽烟，这时老师出现在他们面前。when这时，那时，相当于and then。while正当……的时候，后跟进行时态。where哪里，what什么，均不合题意。因为appear是瞬间动词，不能表示动作的延续，故选D。