Which statement best describes the trend in U.S. air pollutant emissions since the Clean Air Act and a factor contributing to that trend?

• A. Emissions rose for all major pollutants due to economic growth.
• B. Only CO2 emissions declined; all other pollutants stayed the same.
• C. Substantial declines in SO2, NOx, and PM; factors include stricter standards, fuel-switching to natural gas, and cleaner vehicle technologies.
• D. PM declined but SO2 increased due to coal use.

Answer: (C)

The main idea is that air-pollutant emissions in the United States have fallen significantly for several major pollutants since the Clean Air Act, thanks to regulation and advancing technology. The Clean Air Act established enforceable emission standards and drove industry to install pollution controls, switch to cleaner fuels, and improve vehicle technologies. Because of these changes, sulfur dioxide (SO2), nitrogen oxides (NOx), and fine particulate matter (PM) have declined substantially. Scrubbers and other controls on coal-fired power plants cut SO2; switching from coal to natural gas reduced both SO2 and NOx; and stricter vehicle standards, catalytic converters, diesel particulate filters, and cleaner fuels lowered NOx and PM from cars and trucks. This combination of regulatory pressure and cleaner tech explains the downward trend in these pollutants. The other statements don’t fit because they imply either increases or limited changes in emissions, whereas the evidence shows broad, significant declines in the major pollutants listed, driven by the standards and technology implementations described.