— I can’t get through to the general manager’s office.
—The line is busy. Someone the telephone.
A. must have used
B. could use
C. could have used
D. must be using
网友回答
A. must have used
B. could use
C. could have used
(答案→)D. must be using
解析：通过The line is busy“线路正忙。”可知后句的意思应该是“有人一定正在用电话.”表示现在正在用。所以应该用must be using。而A.must have used表示“一定用过”B.表示可能用，不一定是此时；C.表示“可能用过”。所以本题选D。