A-A+
Every possible means ______ to prevent the air
问题详情
Every possible means ______ to prevent the air pollution, but the sky is still not clear.
A) is used B) are used C) has been used D) have been used
参考答案
C主语的means为单数名词，意思是“手段、方法”，因此，要先排除B和D项；根据句意“所有阻止空气污染的方法都用过了，但天空气还是不干净。”，可见，此处应该用现在完成时。因此选C。
