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"[r]easonable," while its Warrant Clause requires warrants to be supported by "probable cause." Nobody would argue that, by expressly making level of suspicion relevant to the validity of a warrant, the Fourth Amendment implicitly makes level of suspicion categorically irrelevant to the reasonableness of a search. To the contrary, all would agree that the expansive word "reasonable" encompasses degree of suspicion alongside other relevant circumstances.) Other parts of the Clean Air Act also expressly mention environmental effects, while §7412(n)(1)(A) does not. Yet that did not stop EPA from deeming environmental effects relevant to the appropriateness of regulating power plants.

Along similar lines, EPA seeks support in this Court’s decision in Whitman v. American Trucking Assns., Inc., 531 U. S. 457 (2001). There, the Court addressed a provision of the Clean Air Act requiring EPA to set ambient air quality standards at levels "requisite to protect the public health" with an "adequate margin of safety." 42 U. S. C. §7409(b). Read naturally, that discrete criterion does not encompass cost; it encompasses health and safety. The Court refused to read that provision as carrying with it an implicit authorization to consider cost, in part because authority to consider cost had "elsewhere, and so often, been expressly granted." 531 U. S., at 467. American Trucking thus establishes the modest principle that where the Clean Air Act expressly directs EPA to regulate on the basis of a factor that on its face does not include cost, the Act normally should not be read as implicitly allowing the Agency to consider cost anyway. That principle has no application here. "Appropriate and necessary" is a far more comprehensive criterion than "requisite to protect the public health"; read fairly and in context, as we have explained, the term plainly subsumes consideration of cost.

Turning to the mechanics of the hazardous-air-