Which cause of this issue is true?

An engineer configured a Cisco AireOS controller with two TACACS+ servers. The engineer notices that when the primary TACACS+ server fails, the WLC starts using the secondary server as expected, but the WLC does not use the primary server again until the secondary server fails or the controller is rebooted.

Which cause of this issue is true?
A . Fallback is enabled
B . Fallback is disabled
C . DNS query is disabled
D . DNS query is enabled

Answer: B