Which command will ensure that External_B will be the primary router for traffic using the gateway
address of 172.16.15.20?

A.
OnExternal_A add the command standby 1 priority 110.

B.
OnExternal_B add the command standby 1 priority 80.

C.
OnExternal_B remove the command standby 1 preempt

D.
OnExternal_A add the command standby 1 priority 80.