He failed in his business a third time and only then ______ the difficult situation he was in.

A．I realized

B．I had realized

C．had I realized

D．did I realize

颈总动脉在哪一平面分为颈外动脉和颈内动脉（）。

A.舌骨大角平面

B.舌骨平面

C.环状软骨平面

D.甲状软骨上缘平面

E.甲状软骨下缘平面