—— 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 using表示此刻“一定正在使用……”。