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

为了防止设备因绝缘损坏而引起外壳还电，造成触电事故，通常采用保护接地措施。保护地线应用一根足够粗的导线与可靠的按地体（）。