The teacher told us that everyone has an important role ________in protecting the earth from________.

A．to play; to pollute

B．to be played; to be polluted

C．to play; being polluted

D．to be played; being polluted

计算机网络的基本分类方法主要有两种：一种是根据覆盖范围与规模；另一种是根据( )。

A．网络协议

B．网络操作系统类型

C．网络所使用的传输技术

D．网络服务器类型与规模