I failed in the last exam and only then the importance of studies.
A. I realized
B. I had realized
C. had I realized
D. did I realize
网友回答
A. I realized
B. I had realized
C. had I realized
(答案→)D. did I realize
解析：考查倒装句。Only +状语放在句首时，应使用部分倒装，根据题中的时间状语是then，故用过去式，助动词did前提，所以选D，句意：上次考试我失败了，只有到了那个时候，我才意识到了学习的重要性。