Question

Yesterday evening I was playing the piano ________ the doorbell rang.

1. A.
   when
2. B.
   before
3. C.
   while
4. D.
   after

Answer 1

A
考查when和while区别。when引导的时间状语从句的谓语动词既可以是延续性的，也可以是终止性的；while引导的时间状语从句的谓语动词只能是延续性动词。ring是终止性动词。故选A。本题稍难。