如图所示是一个电表的表盘．也许你没有见过，但根据已学知识可知，该电表使用前也需校零，观察并明确__________ 和 __________；使用该表所测量的物理量是__________ ，图中示数是__________ ．

Only after Helen moved to Australia ______ how important she was to me .

A．I had realized

B．had I realized

C．I did realize

D．did I realize