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.
- E Each AS must keep routing tables converged to prevent dead routes from being advertised across the AS boundary.