—How much shall I pay for the telephone call?
—You . It's free.
A. mustn't
B. shouldn't
C. don't have to
D. can't
网友回答
A. mustn't
B. shouldn't
(答案→)C. don't have to
D. can't
解析：此题重点考查情态动词的基本用法。Must表示必须，一定，其否定式mustn’t意思的千万不要的意思；should表示应该，其否定形式是shouldn’t表示不该的意思；have to表示必须，其否定式是don’t have to表示不必；can表示可能，能够，其否定形式是can’t，表示不能。根据题干的含义可知选择C