To my disappointment every possible means —— to prevent air pollution butthe sky is stillnotclea
问题描述：
To my disappointment every possible means —— to prevent air pollution butthe sky is stillnotclea
A.are being used
B.have been used
C.has been used
D.had been used
选什么 为什么
答
C means:方式 单复数同形,前面有every,所以这里是单数,又是强调过去动作产生的影响,所以用完成时