Question

【题目】We were having a lesson a policeman came in.
A.when
B.while
C.until
D.before

Answer 1

【答案】A
【解析】句意：我们正在上课，突然一个警察进来了。此处when用作并列连词，表示＂就在那时＂，相当于and at that time。故选A。