Which of the following is a primary pollutant? a. acid rain, which forms when sulfur dioxide or nitrogen oxides react with water b. co, which comes from fossil fuel–powered engines, such as cars c. no2, which forms when no combines with oxygen in the air d. ozone, which forms when oxygen molecules and atoms combine in the presence of sunlight

The correct answer would be B. CO, which comes from fossil fuel–powered engines, such as cars.

As the name suggests the primary pollutant is defined as the pollutant emitted directly from the source. For example, carbon monoxide which is produced due to incomplete combustion is emitted directly from cars, buses et cetera.

In contrast, secondary pollutant refers to the pollutant which is formed after reacting with other pollutants in the atmosphere. For example, nitrogen dioxide which is formed when NO reacts with oxygen, acid rain which is formed when nitrogen dioxide and sulfur dioxide reacts with water present in atmosphere et cetera.