All possible means been tried to stop the air .
A. has, polluted
B. have, polluting
C. have, being polluted
D. has, being polluted
网友回答
A. has, polluted
B. have, polluting
(答案→)C. have, being polluted
D. has, being polluted
解析：第一空考查主谓一致means是单复同形的名词本句中有all视为复数；第二空考查短语stop …(from)…“阻止做某事”空气应是被污染要用动名词的被动式。句意为：已经尝试了所有的方法来阻止空气被污染。选C