Radon is

• A. A colorless gas that is harmless indoors.
• B. A radioactive gas that can enter homes from the ground and increase lung cancer risk.
• C. A pollutant that comes from cars.
• D. A type of ozone.

Answer: (B)

Radon is a radioactive gas produced by the natural decay of uranium in soil. It can seep into homes from the ground through cracks and openings in foundations, especially basements. Because it is radioactive, breathing it over time increases the risk of lung cancer, with higher risk for smokers. This combination of a soil-sourced entry into indoor spaces and a cancer risk from radiation is what makes the statement describing radon as a radioactive gas that can enter homes from the ground and increase lung cancer risk the correct choice.

The other options miss key points: radon is not harmless indoors (it is dangerous despite being colorless), it does not come from cars, and it is not ozone. If you’re studying this topic, testing for radon in homes and using mitigation methods to vent it outdoors are practical steps to reduce exposure.