1. [单选题]TCP23是（）协议端口号(port number)；UDP69是（）端口号(port number)；TCP80是（）端口号(port number)。
A. TELNET FTP HTTP
B. FTPT FTP HTTP
C. TELNET TFTP HTTP
D. TELNET DNS HTTP
2. [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)
A. Each device on the Ethernet segment stops transmitting for a short time.
B. A jam signal informs all devices that a collision occurred.
C. When data transmission resumes, the devices that were involved in the collision have priority to transmit.
D. The devices that are involved in the collision stops transmitting for a short time.
E. The collision invokes a random back-off algorithm.
3. [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（）
A. 端口处于关闭状态
B. 物理端口被烧毁
C. 两端设备协议不一致
D. 路由表中无路由
