All these means to prevent the air pollution, but the sky is still not clear. A. is used ——青夏教育精英家教网—

摘要： All these 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

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All these 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

All these 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

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