It might have been John bought a present for Mary yesterday.A. thatB. whenC. whatD. which

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(答案→)A. that
B. when
C. what
D. which
解析：考查强调句型，it is that……。本句中引导词指代前文内容，只是其中的动词be采用了might have been这一复杂结构。故引导词用that。选A。