Which command will need to be added to External_A to ensure that it will take over if serial 0/0 on External_B fails?
Exhibit

A.
standby 1 priority 130

B.
standby 1 preempt

C.
standby 1 track fastethernet 0/0

D.
standby 1 track 10.10.10.1

Explanation:

You can configure a router to preempt or immediately take over the active role if its priority is the highest at any time. Use the following interface configuration command to allow preemption:
Switch(config-if)# standby group preempt [delay seconds] By default, the router can preempt another immediately, without delay. You can use the delay keyword to force it to wait for seconds before becoming active. This is usually done if there are routing protocols that need time to converge.