Final answer:
Automobiles (C) are the greatest single contributor to urban air pollution, emitting various pollutants that can be harmful to human health.
Step-by-step explanation:
The greatest single contributor to urban air pollution is automobiles (C).
Fossil fuels burned in cars, trains, and planes release air pollutants such as carbon monoxide, nitrogen oxide, and particulate matter.
These pollutants contribute to poor air quality and can be harmful to human health.