If only all network elements in a certain subnet are out of management on the network management system, and network elements belonging to other subnets communicate normally, which of the following reasons is impossible?

If only all network elements in a certain subnet are out of management on the network management system, and network elements belonging to other subnets communicate normally, which of the following reasons is impossible?
A . Changing the IP address of the gateway NE by mistake
B. NMS network management failure
C. Misconnection or failure of optical fiber
D. The network scale is too large, and the scale of ECC communication between network elements exceeds the processing capacity of network elements

Answer: B