Only when I left my parents for America ____ how much I loved them

A.I realized

B.I had realized

C.had I realized

D.did I realize

答案及解析： D

考查倒装。Only+ when引导的时间状语从句放在句首时，主句要部分倒装，所以排除A,D；C项表示意识到我有多爱他们这一动作发生在left之前，不符合句意，所以选D。