单项选择题 While troubleshooting a network connectivity problem, a technician observes steady link lights on both the workstation NIC and the switch port to which the workstation is connected. However, when the ping command is issued from the workstation, the output message "Request timed out." is displayed. At which layer of the OSI model does the problem most likely exist（）。

多项选择题 Which statements describe the routing protocol OSPF? （）

多项选择题 Host 1 is trying to communicate with Host 2. The e0 interface on Router C is down. Which of the following are true (Choose two.)（）。