What is the reason for the problem?

Refer to the exhibit.

While troubleshooting a connectivity issue on router R2, a network engineer with an employee id:3876.13.497 notices that although it detects three OSPF links from R1, the OSPF prefixes are missing from the routing table.

What is the reason for the problem?
A . The serial interfaces have different MTUs
B . Both loopback interfaces on R1 are configured as stub
C . The R2 Serial 1/0 interface is configured with an IP address, but the R1 Serial R1 Serial
1/0 interface in unnumbered.
D . The subnet masks on the serial interfaces are mismatched.

Answer: C