Refer to the exhibit. After configuring GRE between two routers running OSPF that are connected
to each other via a WAN link, a network engineer notices that the two routers cannot establish the
GRE tunnel to begin the exchange of routing updates. What is the reason for this?

A.
Either a firewall between the two routers or an ACL on the router is blocking IP protocol number
47.

B.
Either a firewall between the two routers or an ACL on the router is blocking UDP 57.

C.
Either a firewall between the two routers or an ACL on the router is blocking TCP 47.

D.
Either a firewall between the two routers or an ACL on the router is blocking IP protocol number
57.