Every possible means _____ to prevent the pollution,but the sky is still not clear.

题目

Every possible means _____ to prevent the pollution,but the sky is still not clear.
A is used B are used C has been used D have been used
A,C 该如何判别?

答案

根据后面说天空依旧不清澈,说明前句的动作已经发生了,用现在完成时的C