Radon is
a. a secondary air pollutant
b. green in color and odorless
c. a substance that seeps into homes and buildings
d. created by the combustion of coal

C. A substance that seeps into homes and buildings.

Air pollution is caused by release of wastes, which can be in the form of solid, liquid and gas. These wastes contaminate the air. These pollutants affects the health of plants, animals and humans. Radon is a odorless and colorless gas. It is a harmful air pollutant. It is produced by radium's radioactive decay in the soil. Radon gas gets mixed with the dust and other particles present in air. It is a dense gas, which sinks in the air. It is often found in the basements of houses, especially where shales and boulders are used for the construction. It seeps inside the house interior and acts as indoor air pollutant. The gas when enters inside the human lungs through respiration. Inside the lungs, radon produces alpha and beta radiation, which damages DNA, and results in lung cancer.  

a time line cause it goes from 685 to 978