EPA’s Clean Power Plan is a rational, solid rulemaking designed to deliver flexible, efficient control of greenhouse gas emissions from existing fossil fuel-fired power plants. EPA has identified the best system of emission reduction that allows states and companies to adjust to locally relevant factors and generation-fleet characteristics, deploying the emission-reduction strategies most appropriate and effective. The Clean Air Act allows a system-based approach under §111 and, in fact, this approach is optimal in satisfying the statutory requirements by securing the cuts in carbon pollution that are needed and doing so through locally appropriate and innovative solutions.