The top student failed again in the examination and only then the importance of reading and memorizing the English texts.
A. he realized
B. he had realized
C. had he realized
D. did he realize
网友回答
A. he realized
B. he had realized
C. had he realized
(答案→)D. did he realize
解析：考查only的倒装句，当only放在句首，且强调状语的时候，后面的句子使用部分倒装的形式。本题的only放在了第二个并列句的句首，强调的状语then，故后面的句子使用部分倒装的形式。故D正确。