All possible means to prevent the air pollution, but the sky is still not clear.
A. is used
B. has been used
C. are use
D. have been used
网友回答
A. is used
B. has been used
C. are use
(答案→)D. have been used
解析：句意：所有可能能阻止大气污染的方法都试过了 ，但是空气依然不干净。means单复数同形，在这里是复数,这里强调过去的动作的影响所以用现在完成时更好。故选D。
考点: 考查动词时态的用法。