Which of the following statements is true regarding the Clean Air Act?

The statement regarding the Clean Air Act that is true is that it established a cap-and-trade program for sulfur dioxide (SO2) in 1990. This program was a critical component of the amendments made to the Clean Air Act to address acid rain, which was primarily caused by emissions of SO2 and nitrogen oxides (NOx) from power plants and industrial sources. The cap-and-trade system allowed for a reduction of SO2 emissions through a market-based approach, where companies could buy and sell allowances for a certain amount of emissions, thereby providing economic incentives for reducing pollution.

The Clean Air Act has undergone several amendments since its original passage in 1963, particularly in 1970, 1977, and 1990, which makes the claim that it has never been modified inaccurate. While the Act does regulate various air pollutants, it does not have direct regulation of carbon dioxide (CO2) emissions from power plants in the same manner as other pollutants prior to the Supreme Court ruling in 2007, which stated CO2 is a pollutant under the Act. Thus, only the establishment of the cap-and-trade program in 1990 is a definitive and accurate fact within the context of the Clean Air Act.