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we need not address—it plainly did not do so here. In the Agency’s own words, the administrative record "utterly refutes [the] assertion that [ancillary benefits] form the basis for the appropriate and necessary finding." 77 Fed. Reg. 9323. The Government concedes, moreover, that "EPA did not rely on the [regulatory impact analysis] when deciding to regulate power plants," and that "[e]ven if EPA had considered costs, it would not necessarily have adopted . . . the approach set forth in [that analysis]." Brief for Federal Respondents 53–54.

We hold that EPA interpreted §7412(n)(1)(A) unreasonably when it deemed cost irrelevant to the decision to regulate power plants. We reverse the judgment of the Court of Appeals for the D. C. Circuit and remand the cases for further proceedings consistent with this opinion. It is so ordered.