The man saw it __ when he was walking past.

问题描述:

The man saw it __ when he was walking past.
The man saw it __ when he was wacking .
A.happen B.happening
为什么B不行
这几个回答不是多满意:::请专业的给与更合理的解释,

看context,如果是看见正在发生的事情,我觉得可以选b