Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Outdoor air pollution is made of chemical particles. When smoke or other pollutants enter the air, the particles found in the pollution mix with the air. Air is polluted when it contains many large toxic particles. Outdoor air pollution changes the natural characteristics of the atmosphere. Primary pollutants are added directly to the atmosphere. Fires add primary pollutants to the air. Particles released from the fire directly enter the air and cause pollution ( Figure below ). Burning of fossil fuels such as oil and coal is a major source of primary pollutants ( Figure below ). Answer: fossil fuels
A major source of primary pollutants is the burning of oil and coal, which are types of what?