Not until I began to work ________ how much time I had wasted.  [ ]

A. did I realize                  

B. didn't I realize

C. hadn't I realized          

D. had I realized

闸阀不适合用于（）的场合。

A、接通介质流量

B、阻断介质流量

C、调节介质流量

D、改变介质流向