问题
选择题
Every possible means ________ to prevent the air pollution，but the sky is still not very clear.
A．were using
B．have used
C．has been used
D．was used
答案
答案：C
答案 C [考查主谓一致及动词时态语态。means意思是“方式”，被every修饰，谓语动词用单数，排除A、B两项；but后的分词用了一般现在时，故此处用现在完成时的被动语态。]