I can't get through to the general manager's office anyhow.

问题描述:

I can't get through to the general manager's office anyhow.
The line is busy.Someone____the telephone.
A.must use B.uses C.must have been using D.must be using
这里的Someone应该是主动的,那为什么答案是D呢?

这里的must be using并不是被动语态,而是推测某人在某个时间一定正在做某事,因为must是情态动词,后面跟动词原形,所以is using就要变成be using