Which of the following steps should be followed to terminate the instance automatically once the job is finished?

A user wants to use an EBS-backed Amazon EC2 instance for a temporary job. Based on the input data, the job is most likely to finish within a week.

Which of the following steps should be followed to terminate the instance automatically once the job is finished?
A . Configure the EC2 instance with a stop instance to terminate it.
B . Configure the EC2 instance with ELB to terminate the instance when it remains idle.
C . Configure the Cloud Watch alarm on the instance that should perform the termination action once the instance is idle.
D . Configure the Auto Scaling schedule activity that terminates the instance after 7 days.

Answer: C

Explanation:

Auto Scaling can start and stop the instance at a pre-defined time. Here, the total running time is unknown. Thus, the user has to use the CloudWatch alarm, which monitors the CPU utilization. The user can create an alarm that is triggered when the average CPU utilization percentage has been lower than 10 percent for 24 hours, signaling that it is idle and no longer in use. When the utilization is below the threshold limit, it will terminate the instance as a part of the instance action.

Reference: http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/UsingAlarmActions.html