Which statement describes the rule of split horizon?（）

A. Only routers can split boundaries (horizons) between concentric networks.
B. All distance vector protocols require fall back routers that may cause momentary loops as the topology changes.
C. Networks can only remain fully converged if all information about routers is sent out all active interfaces.
D. Information about a route should not be sent back in the direction from which the original update come.
Each AS must keep routing tables converged to prevent dead routes from being advertised across the AS boundary.