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？（）

问答题 Routers R1， R2， and R3 are configured in a hub and spoke frame relay environment， with router R1 as the hub. You have configured Router R1， Router R2， and Router R3 to run EIGRP over the frame relay connections. No sub-interfaces are used. You have configured a single IP subnet on all the Frame Relay interfaces. Router R1 can reach both router R2 and R3， but R2 and R3 can not reach each other. What is the probable cause of this problem？

单项选择题 Refer to the exhibit. Routers A and B are directly connected and running EIGRP，but they are unable to form a neighbor relationship. What is the most likely cause？（）