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.
