WHO推荐癌痛治疗首选的阿片类药物是（）。

A.喷他佐辛

B.美沙酮

C.吗啡

D.哌替啶

E.可待因

— How do you usually go to work?

— If it is fine, I _____ on foot. [ ]

A. will go

B. go

C. have gone

D. won't go