You are deploying an application to App Engine. You want the number of instances to scale based on request rate. You need at least 3 unoccupied instances at all...

You are deploying an application to App Engine. You want the number of instances to scale based on request rate. You need at least 3 unoccupied instances at all times.
Which scaling type should you use?

A. Manual Scaling with 3 instances. B. Basic Scaling with min_instances set to 3. C. Basic Scaling with max_instances set to 3. D. Automatic Scaling with min_idle_instances set to 3.