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
在正方体ABCD﹣A1B1C1D1的各面的12条对角线中,与正方体的对角线A1C垂直的共有____ 条.
15 20 25 ( ) 。