注意力缺陷多动症首选的治疗药物为()

A.苯巴比妥

B.左旋多巴

C.哌醋甲酯（利他林）

D.地西泮

E.ACTH

Though most people know that smoking ________ the risk of cancer, it is still hard for them to give it up.[ ]

A. multiplies 

 B. adds

C. combines  

D. gains