—— 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
网友回答
A. must use
B. uses
C. must have been using
(答案→)D. must be using
解析：must be doing 表示对正在发生的事情的推测。 “我怎么也打不通经理的电话。”“线路忙，肯定有人在用电话”