When does the billing of an Amazon EC2 system begin?

When does the billing of an Amazon EC2 system begin?
A . It starts when the Status column for your distribution changes from Creating to Deployed.
B . It starts as soon as you click the create instance option on the main EC2 console.
C . It starts when your instance reaches 720 instance hours.
D . It starts when Amazon EC2 initiates the boot sequence of an AMI instance.

Answer: D

Explanation:

Billing commences when Amazon EC2 initiates the boot sequence of an AMI instance. Billing ends when the instance terminates, which could occur through a web services command, by running "shutdown -h", or through instance failure. When you stop an instance, Amazon shuts it down but doesnÆt charge hourly usage for a stopped instance, or data transfer fees, but charges for the storage for any Amazon EBS volumes.

Reference: http://aws.amazon.com/ec2/faqs/