物理层的主要功能是利用物理传输介质为数据链路层提供物理连接,以便透明地传送【 】

A．比特流

B．帧序列

C．分组序列

D．包序列

.

I have told her never to write to me again afterwards .

You______be sorry for what you said to her .

A．need

B． shall

C．have to

D．ought to