Tom have kept his word. I wonder why he changed his mind.
A. must B. should C. need D would
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解析：考查情态动词和虚拟语气。句意：汤姆本应该遵守承诺的。我想知道他为什么改变主意。must have done一定已经做了某事；should have done本应该做某事；need have done本来需要做某事；would have done用于对过去事情的虚拟。根据句意可知，是汤姆没有遵守承诺，用should表示“本应该”的意思。故选B。