问题补充：
To keep the air clean this hospital forbids all the patients in and around it.
A. smoke
B. smoking
C. to be smoking
D. smoked
答案：
A. smoke
(答案→)B. smoking
C. to be smoking
D. smoked
解析：forbid sb to do/forbid doing (doing 前可加逻辑主语all the patients )
