—How much shall I pay for the phone call?
—You . This is free of charge.
A. shouldn't
B. can't
C. don't have to
D. mustn't
网友回答
A. shouldn't
B. can't
(答案→)C. don't have to
D. mustn't
解析：考查情态动词。shouldn’t不应该；can’t不可能；don’t have to 不需要；mustn’t不允许。句意是---这个电话我应该付多少钱？---你不需要付钱。这是免费电话。故选C。