新概念英语上的问题.

问题描述:

新概念英语上的问题.
_,it immediately ran away.
a.Observing her b.On being observed
c.Having been observed d.On her being observed
这道题为什么选b?其他几个为什么不选?

逗号前后分为两个部分,但是这是一个句子,所以要连词连接前后两个句子,A和B排除。b应该是固定搭配,被观察到。