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

问题补充：

You buy a gift, but you can if you want to.

A. must

B. mustn

C. have to

D. don have to

答案：

A. must

B. mustn

C. have to

(答案→)D. don 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。