Which circumstances can cause a GRE tunnel to be in an up/down state?

A.	The tunnel interface IP address is misconfigured.
B.	The tunnel interface is down.
C.	A valid route to the destination address is missing from the routing table.
D.	The tunnel address is routed through the tunnel itself.
E.	The ISP is blocking the traffic.
F.	An ACL is blocking the outbound traffic.