The correct answer is A. SO2.
Primary pollutants are pollutants that are emitted directly into the air from a source, such as a power plant or a car. They can be natural or man-made.
SO2 is sulfur dioxide, a gas that is released into the air when fossil fuels are burned. It is a major contributor to acid rain and smog.
NO2 is nitrogen dioxide, a gas that is also released into the air when fossil fuels are burned. It is a major contributor to ground-level ozone.
CO is carbon monoxide, a gas that is released into the air when fossil fuels are burned. It is a colorless, odorless gas that can be deadly if inhaled in high concentrations.
Hydrocarbons are a group of chemicals that are found in gasoline and other petroleum products. They are released into the air when gasoline is burned in cars and other vehicles. They can contribute to smog and ground-level ozone.
In conclusion, the correct answer is A. SO2.