Question

I was walking through the street ________ I met an old friend of mine.

A. when
B. while
C. during
D. for

Answer 1

答案A
剖析：while引导时间状语从句，谓语动词必须是持续性动词．此空应填并列连词when，意思是“就在那时”，表示动作的突然性，引出前一个分句的动作正在进行时，意想不到所发生的事情．