下列哪种气体被认为是全球气候变暖的原因（　　）

A．二氧化碳

B．氧气

C．臭氧

D．一氧化碳

A special device that，when fitted into the rotary table，transmits torque to the kelly and simultaneously permits vertical movement of the kelly to make the hole is called a（）.

A.master bushing

B.kelly slips

C.kelly bushing

D.rotary bushing