The driver have drunk a lot before the accident, but I'm not quite sure of thatA. mightB

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(答案→)A. might
B. must
C. should
D. would
解析：考察情态动词用法。Might have done也许做了某事；must have done一定做了某事；should have done本应该做某事，实际上却未做；would have done会做某事；句意：司机也许在事故之前喝酒了。但是我也我不确定。根据后半句我不确信，说明这是对过去情况的一种没有把握的推测。故A正确。