单项选择题 Whiletroubleshootinganetwork，youneedtoverifythelivenessofhostsinthesubnet192.168.1.64/26.Allofthehostsareabletoreplytopingrequests.Howwouldyouconfirmtheexistingnodesusingonesinglecommand？（）

单项选择题 Whichmechanismcanyouusetoachievesub-secondfailoverforlinkfailuredetectionwhenaswitchedEthernetmediaisusedandlossofsignalisnotsupportedbythelinkprovider？（）

单项选择题 Refer to the exhibit. R2 and R3 are routers connected using Ethernet services from a service provider and can receive pings from each other. OSPF is configured as the routing protocol but adjacency is not happening. According to the output of the show commands in the exhibit, what could be the most likely cause of the problem?（）