After Making a Configuration Change to a Gateway Router, All

After making a configuration change to a gateway router, all of the devices past that router appear down in the network management system (NMS) . However, the NMS is still gathering CPU and memory statistics on the devices. What is the first thing you should do to diagnose the issue?

A) check the configurations of each affected device
B) check the gateway router configuration for an access list change
C) check the devices to see if any antivirus is blocking ICMP
D) check the gateway router configuration to make sure a default route exists