Refer to the exhibit. A company has a requirement to provide communication among remote sites via multipoint GRE tunnels. This communication is not working yet. Which is the root cause of this issue?
A. The key that is used on the DMVPN tunnel between the spoke and hub ends does not match. This causes the tunnel to fail.
B. BGP IPv4 multicast address-family is missing between hub and spoke points.
C. The GRE tunnel is not sourced from a physical interface. The DMVPN traffic does not know where to forward packets.
D. The NHRP network ID does not match in the hub and spoke configuration causing NHRP negotiations to fail and the mGRE to stay down.
