问题补充:
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
答案:
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。