You work for a Defense contracting company. The company develops software applications which perform intensive calculations in the area of Mechanical Engineering related to metals for ship building. The company competes for and wins contracts that typically range from 1 year to up to 5 years. These long-term contracts mean that the duration of your need for EC2 instances can be matched to the length of these contracts, and then extended if necessary. The main requirement is consistent performance for the duration of the contract. Which EC2 purchasing option provides the best value, given these long-term contracts?