Every possible means ________ to prevent water pollution, but we still have not enough clean water．A.has been usedB.are usedC.were usedD.have been used
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A解析考查时态和主谓一致。means前有every修饰，谓语动词只能用单数，且根据语境we still have not enough clean water可知到现在为止我们已经使用了每一种可能的方法，故选A。