He failed the mid-term examination and only then how much time he had wasted.
A. he realized
B. did he realize
C. he had realized
D. had he realized
网友回答
A. he realized
(答案→)B. did he realize
C. he had realized
D. had he realized
解析：句意：他期中考试不及格，只有到了那时，他才意识到他浪费了多少时间。
Only+副词/介词短语/状语从句等，后面的句子主体部分用倒装结构，即助动词+主语+动词原形，因此排除A,C。又根据句意，不是先意识到浪费时间了，然后考试不及格，故排除D。故选B。