Question

Last Saturday, we were walking along the street ______ a car suddenly knocked an old man over.

1. A.
   when
2. B.
   while
3. C.
   as
4. D.
   since

Answer 1

A
本题着重考察when与while和as的区别。when可以和延续性动词连用，也可以和短暂性动词连用；而while和as只能和延续性动词连用。 如：Why do you want a new job when you have got such a good one already？（get为短暂性动词,不可用while或as）你已经找到如此好的工作，为何还想再找新的？ 本题从句中knock为短暂性动词，故选A正确。