1. [单选题]OSI七层模型中，“包”是哪一层数据封装的名称（）
A. 物理层
B. 数据链路层(data link layer)
C. 网络层
D. 传输层
2. [单选题]On the topic of VLSM, which one of the following statements best describes the concept of the route aggregation?
A. Deleting unusable addresses through the creation of many subnets.
B. Combining routes to multiple networks into one supernet.
C. Reclaiming unused space by means of changing the subnet size.
D. Calculating the available host addresses in the AS.
E. None of the above
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
