He failed in the exams 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：考查ｏｎｌｙ的倒装句，ｏｎｌｙ接状语的时候，谓语动词用部分倒装，ｔｈｅｎ这里是时间状语，句意：当他考试失败了，只有那时他才意识到浪费了多少时间。时态应该是一般过去时，选Ｂ。