[单选题]TCP23是（）协议端口号；UDP69是（）端口号；TCP80是（）端口号。

正确答案 ：C

TELNET TFTP HTTP

[多选题]Two stations on the Ezonexam LAN transmit at the same time, resulting in a collision. What happens when a collision occurs on the network? (Choose all that apply)

正确答案 ：BDE

A jam signal informs all devices that a collision occurred.

The devices that are involved in the collision stops transmitting for a short time.

The collision invokes a random back-off algorithm.

解析：解析：Explanation:

[单选题]路由器上，命令“show int s0”的显示结果为：Serial0 is up,line protocol is down出现该结果的可能原因是（）

正确答案 ：C

两端设备协议不一致

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