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pollutants] after imposition of the requirements of this chapter.” §7412(n)(1)(A). If the Agency “finds. . . regulation is appropriate and necessary after considering the results of the study,” it “shall regulate [power plants] under [§7412].” Ibid. EPA has interpreted the Act to mean that power plants become subject to regulation on the same terms as ordinary major and area sources, see 77 Fed. Reg. 9330 (2012), and we assume without deciding that it was correct to do so.

And what are those terms? EPA must first divide sources covered by the program into categories and subcategories in accordance with statutory criteria. §7412(c)(1). For each category or subcategory, the Agency must promulgate certain minimum emission regulations, known as floor standards. §7412(d)(1), (3). The statute generally calibrates the floor standards to reflect the emissions limitations already achieved by the best-performing 12% of sources within the category or subcategory. §7412(d)(3). In some circumstances, the Agency may also impose more stringent emission regulations, known as beyond-the-floor standards. The statute expressly requires the Agency to consider cost (alongside other specified factors) when imposing beyond-the-floor standards. §7412(d)(2).

EPA completed the study required by §7412(n)(1)(A) in 1998, 65 Fed. Reg. 79826 (2000), and concluded that regulation of coal-and oil-fired power plants was “appropriate and necessary” in 2000, id., at 79830. In 2012, it reaffirmed the appropriate-and-necessary finding, divided power plants into subcategories, and promulgated floor standards. The Agency found regulation “appropriate” because (1) power plants’ emissions of mercury and other hazardous air pollutants posed risks to human health and the environment and (2) controls were available to reduce these emissions. 77 Fed. Reg. 9363. It found regulation “necessary” because the imposition of the Act’s other