You buy a gift, but you can if you want to.A. mustB. mustn'tC. have toD. don't have to

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A. must
B. mustn't
C. have to
(答案→)D. don't have to
解析：don’t have to 没有必要做某事儿，mustn’t 则为禁止。句意：你没有必要买礼物，但是如果你想的话，你可以买。must 必须, mustn’t表禁止 have to 不得不。