All possible means ____ to prevent the air pollution, but the sky is still not clear.

A．is used B．has been used C．a(chǎn)re use D．have been used 

D

解析試題分析：句意：所有可能能阻止大氣污染的方法都試過了 ，但是空氣依然不干凈。means單復數(shù)同形，在這里是復數(shù),這里強調(diào)過去的動作的影響所以用現(xiàn)在完成時更好。故選D。
考點: 考查動詞時態(tài)的用法。

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