_____quite common for people to believe that more and more teenagers have become addicted to on line games.

A．As is

B．That is

C．It is

D．What is 

依据《国家电力监管委员会安全生产令》，为提高电力企业职工安全素质，应加强（）培训。

A.安全生产知识

B.安全生产法律法规

C.生产技术

D.安全管理理论