单项选择题 Which layer in the OSI reference model is responsible for determining the availability of the receiving program and checking to see if enough resources exist for that communication?（）

单项选择题 A company implements video conferencing over IP on their Ethernet LAN. The users notice that the network slows down, and the video either stutters or fails completely. What is the most likely reason for this?（）

单项选择题 指定逻辑通道备份主接口使用的命令是（）。