Which substance has historically been targeted by cap-and-trade programs to reduce acid deposition?

• A. Ozone.
• B. Sulfur dioxide.
• C. Carbon dioxide.
• D. Particulate matter.

Answer: (B)

The key idea is that cap-and-trade programs were designed to cut the pollutants most responsible for a problem, and for acid deposition that pollutant is sulfur dioxide. When fossil fuels, especially coal, are burned, sulfur dioxide is released and can react in the atmosphere with moisture to form sulfuric acid, which then falls as acid rain. Reducing SO2 emissions directly lowers the amount of acid that reaches lakes, soils, and forests, making it the pollutant targeted by early cap-and-trade efforts like the Acid Rain Program. While other pollutants like ozone, carbon dioxide, or particulate matter are important for different environmental or health reasons, they are not the primary culprits behind acid deposition, which is why sulfur dioxide is the best fit here.