Though this question is very difficult, if you ________ trying you will find the right answer.[ ]

A. keep up

B. keep on

C. keep out

D. keep off

阿片类物质依赖的非替代治疗主要使用以下哪种药物（）

A．可乐定

B．中草药

C．丁丙诺啡

D．苯二氮类药

E．纳屈酮