There are two main sets of Games — the Winter and the Summer Olympics, and both are held every four years on a _________ basis.­­­­­

A．valuable

B．former

C．regular

D．swift

医院的Ⅱ类环境的空气消毒，可选用（）。

A.循环风紫外线空气消毒器消毒

B.普通紫外线消毒

C.静电吸附式空气消毒器消毒

D.臭氧消毒

E.机械通风消毒