－How was your final exam, Tom?

－I’m sorry, dad. I failed and only then       the importance of studies.

A．I realized

B．I had realized

C．had I realized

D．did I realize

吗啡对中枢神经系统的药理作用包括

A．镇痛、镇静

B．镇咳

C．抑制呼吸

D．瞳孔缩小

E．胆道平滑肌痉挛