Coal-fired generating plants are a principal source of atmospheric sulfur dioxide emissions. The CAA requires newly constructed generating units to use systems of emissions control approved by the EPA. The EPA initially provided two methods for controlling sulphur dioxide emissions: (1) the use of low-sulfur coal; and (2) the installation of a device to scrub high-sulfur coal emissions before they reach the atmosphere. Scrubbing is costlier than using low-sulfur western coal. States producing high-sulfur coal suffered competitively. In 1990, new, even more, strict emissions standards were adopted. Utilities now have a choice of the following strategies: (1) installing pollution control devices; (2) using low-sulfur coal; (3) purchasing allowances to emit sulfur dioxide; (4) switching to another fuel; (5) closing down certain units; (6) offsetting emissions at one plant by over-complying at another; or (7) adopting some combination. High-sulfur coal-mining states like Indiana were facing trouble. D adopted its ECPA (Environmental Compliance Plans Act) to comply with the standards and also make sure the plans provide for continued or increased use of Indiana coal. P sued challenging the Act. P moved for summary judgment. The court concluded that the ECPA was intended to promote high sulfur coal at the expense of western coal and unconstitutionally burdens interstate commerce. D appealed.