问题：

[单选] 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.