Refer to the exhibit. A ping between the serial interfaces of R1 and R2 is successful, but a ping between their FastEthernet interfaces fails. What is the reason for this problem?

When serial interfaces can ping each other but FastEthernet interfaces cannot, the issue is typically incorrect default route configuration. One router's default route likely points to the wrong next-hop address or interface, causing packets destined for the FastEthernet network to be sent incorrectly. If FastEthernet interfaces were disabled, the serial ping would also likely fail. A routing protocol is not necessary if static routes are configured. Default gateway configuration applies to hosts, not routers.