Which one of the following statements best explains the split horizon rule used in distance vector routing protocols?（）
- A Only routers can split boundaries (horizons) between concentric networks.
- B Each AS must keep routing tables converged to prevent dead routes from being advertised across boundaries.
- C Networks can only remain fully converged if all information is sent out all active interfaces.
- D Information about a route should not be sent back in the direction from which the original update came.
- E Distance vector protocols need fall back routers that are responsible for momentary loops.