To keep the air clean this hospital all the patients in and around it.A. allows; smokin

问题补充：

To keep the air clean, this hospital all the patients in and around it.

A. allows; smoking

B. forbids; to smoke

C. prevents; smoke

D. prohibits; smoke

答案：

A. allows; smoking

(答案→)B. forbids; to smoke

C. prevents; smoke

D. prohibits; smoke

解析：考查固定词组。固定词组forbid sb to do sth禁止某人做某事； ACD的搭配错误或者与句意不符的。句意：为了保持空气清洁，这家医院禁止病人在内外抽烟。故B正确。