Final answer:
The greatest single source of air pollution in the United States is motor vehicles, specifically the burning of fossil fuels in cars, trucks, and other vehicles.
Step-by-step explanation:
The greatest single source of air pollution in the United States is motor vehicles, specifically the burning of fossil fuels in cars, trucks, and other vehicles. Vehicle emissions, which include carbon monoxide, nitrogen oxides, and particulate matter, contribute significantly to air pollution and the formation of smog. According to a study by MIT, vehicle emissions are responsible for tens of thousands of premature deaths in the United States each year.