Only then the importance of my health.A. did I realize B. had I realized C. I realized D.

问题补充：

Only then the importance of my health.

A. did I realize

B. had I realized

C. I realized

D. I had realized

答案：

(答案→)A. did I realize

B. had I realized

C. I realized

D. I had realized

解析：Only加状语置于句首，用部分倒装。Only+状语+助动词+主语+动词+其他。