Answer:
C. Indoor
Explanation:
Radon is a radioactive noble gas generated in soils that are enriched in uranium, thorium and radio. These natural elements can be found anywhere in the world, so any building is subject to high levels of radon. The higher the content of radio in the soil, the greater the potential for high levels of radon to appear inside buildings built above the ground, so we can consider that radon causes indoor pollution.
