Which one of the following statements best explains the split horizon rule?（）
- A Only routers can split boundaries (horizons) between networks in separate AS numbers.
- B Each AS must keep routing tables converged to prevent dead routes from being advertised across boundaries.
- C Once a route is received on an interface, advertise that route as unreachable back out the same interface.
- D Information about a route should never be sent back in the direction from which the original update came.