Question

what is the most effective approach the government can take to keep the air clean?
create and enforce regulations that limit fossil fuel burning by industries
use funds to understand why smoke and mold are causes of outdoor air pollutants
educate people about the threats to outdoor clean air from visible forms of air pollution
modify regulations under the clean air act by including expensive options to reduce air pollution

Explanation:

Limiting fossil - fuel burning by industries directly reduces major air pollutants. Understanding causes (second option) is research - oriented, education (third option) is awareness - raising but not a direct control measure, and modifying regulations with expensive options (fifth option) may not be the most practical or effective immediately.

Answer:

create and enforce regulations that limit fossil fuel burning by industries