Only when I left my parents for Beijing ______ how much I loved

Only when I left my parents for Beijing ______ how much I loved them.
A、I realized
B、I had realized
C、did I realize
D、had I realized
【正确答案】：C
【题目解析】：Only 提前要引起倒装，realized 是谓语动词，因此借用did助动词