Refer to the exhibit. Routers East and West are configured using RIPv1. Both routers are sending updates about their directly connected routes. The East router can ping the West router serial interface and West can ping the serial interface of East. However, neither router has dynamically learned routes from the other. What is most likely the problem?

Refer to the exhibit. Routers East and West are configured using RIPv1. Both routers are sending updates about their directly connected routes. The East router can ping the West router serial interface and West can ping the serial interface of East. However, neither router has dynamically learned routes from the other. What is most likely the problem?
A gateway of last resort is required.
Subnetting is not supported by RIPv1.
VLSM is not supported by RIPv1.
One of the routers needs a clock rate on the serial interface.