【选择题】Only when i left my parents for Italy ___ how much i loved them.

【选择题】Only when i left my parents for Italy ___ how much i loved them.
A.i realized
B.i had realized
C.had i realized
D.did i realize
为什么呀,看不清.
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为什么不用C，完成时呢？
注意：为什么不能用过去完成时？