What is the second-leading cause of cancer,after smoking? 

A)  nitrous oxide exposure 
B)  formaldehyde exposure 
C)  radon exposure 
D)  sulphur dioxide exposure

Radon exposure is the second-leading cause of lung cancer after smoking. Radon is a radioactive gas that is produced by the natural decay of uranium in soil, rock, and water. It can seep into homes and buildings, where it can accumulate to dangerous levels. People who are exposed to high levels of radon over a long period of time are at increased risk of developing lung cancer. Nitrous oxide exposure is not a known cause of cancer, and formaldehyde and sulphur dioxide exposure are classified as possible carcinogens but are not as strongly linked to cancer as radon.