Final answer:
In 2004, Ohio, Pennsylvania, and Indiana were the top three states with the highest SO2 emissions, reflecting their historical industrial activities and reliance on coal power.
Step-by-step explanation:
The three states that led the nation in SO2 emissions in 2004 were Ohio, Pennsylvania, and Indiana. This fact is indicative of the broader issue of air pollution that has been historically high in certain regions due to industrial activities. For instance, the Cuyahoga River in Ohio was famously so polluted that it caught fire in 1969.
Furthermore, states like Ohio and Pennsylvania have a history of heavy industrialization and coal power usage, which contributed to higher sulfur dioxide emissions. These emissions, which can travel across state and national borders, illustrate the challenges in managing and reducing air pollution.