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

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A. I realized
B. I had realized
C. had I realized
(答案→)D. did I realize
解析：考查only的倒装句：only+状语（介词短语，副词，状语从句）后面的主句要部分倒装，这句话only then是时间状语，后面的句子倒装，因为是同时发生，所以是过去时，did提到主语前面，句意：期末考试我不及格，只有那时我才知道学习多么的重要。选D。