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.