问题
选择题
Not until he failed the exam _______he had left his books untouched .
A．did he realize
B．had he realized
C．he realized
D．he had realized
答案
答案：A
题目分析：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。