I failed in the final examination last term and only then the importance of studies.A. I r

问题补充：

I failed in the final examination last term and only then the importance of studies.

A. I realized

B. I had realized

C. had I realized

D. did I realize

答案：

A. I realized

B. I had realized

C. had I realized

(答案→)D. did I realize

解析：考查only的部分倒装。Only引导的部分倒装要满足两个条件：only放在句首，且强调的是状语。句意：在上学期的期末考试中我没有及格，就在那时我才意识到学习的重要性。根据句意说明是在过去发生的事情，故使用一般过去时。故D正确。