What could explain this connectivity problem?

Refer to the exhibit. S0/0 on R1 is configured as a multipoint interface to communicate with R2
and R3 in this hub-and-spoke Frame Relay topology. While testing this configuration, a technician
notes that pings are successful from hosts on the 172.16.1.0/24 network to hosts on both the
172.16.2.0/25 and 172.16.2.128/25 networks. However, pings between hosts on the 172.16.2.0/25
and 172.16.2.128/25 networks are not successful. What could explain this connectivity problem?

A.
The RIP v2 dynamic routing protocol cannot be used across a Frame Relay network.

B.
The ip subnet-zero command has been issued on the R1 router.

C.
Split horizon is preventing R2 from learning about the R3 networks and R3 from learning about
the R2 networks.

D.
The 172.16.3.0/29 network used on the Frame Relay links is creating a discontiguous network
between the R2 and R3 router subnetworks.

E.
The 172.16.2.0/25 and 172.16.2.128/25 networks are overlapping networks that can be seen by
R1, but not between R2 and R3.

Explanation:
Under normal circumstances, the router that is connected to the broadcast IP network and uses

the distance vector routing protocol will use split horizon mechanism to avoid routing loop.
Split horizon is a technology to avoid routing loop and speed up the routing convergence. The
router may receive the routing information sent by itself which is useless, split horizon will not
advertise the routing update information back received from the terminal , while it will advertise
those routes that will not be cleared because of the endless counting .It can be simply interpreted
that the route learnt by a router from one interface will not be sent through the same interface .
For the non-broadcast network (such as Frame -Relay and High Speed Switched Data Services),
the effect of split horizon is not ideal. So, we can use the following commands to disable or enable
split horizon.
ip split-horizon to enable split horizon