Not until he failed the exam he had left his books untouched .A. did he realizeB. had he r

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(答案→)A. did he realize
B. had he realized
C. he realized
D. he had realized
解析：句意：直到他考试失败了，他才意识到他没有碰书本。Not until+时间状语从句，放在句首的时候，后面的主句用部分倒装，助动词放在谓语前面，因为realize是发生在fail后面，不是前面，不能用过去完成时，用一般过去式，选A。