The line was busy. Someone（）the telephone.

The line was busy. Someone（）the telephone.


A、

must be using


B、

must use


C、

must have been using


D、

should have been using


【正确答案】：C
【题目解析】：

“线路正忙（那会儿），一定是有人正在用电话。”对过去正在进行的事情进行肯定的推测，用must+ have been doing.