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.