Question

which factor is a major contributor to outdoor air pollution?
radon
fossil fuels
ultrafine particles
carbon monoxide

Explanation:

Fossil fuels, when burned in vehicles, power - plants, etc., release a large amount of pollutants like sulfur dioxide, nitrogen oxides, and particulate matter into the outdoor air. Radon is more of an indoor pollutant. Ultrafine particles are a part of the pollution but are often a result of fossil - fuel combustion. Carbon monoxide is also a pollutant from fossil - fuel burning but fossil fuels are the overarching major contributor.

Answer:

B. fossil fuels