He was about to go out for a walk _____ it begin to rain.

问题描述:

He was about to go out for a walk _____ it begin to rain.
a.when b.while c.as
选B 为什么?A.C 为何不可

答案是B
固定巨型:be about to do sth when +句子,这是 when 的最典型用法