Fact 1: Lung cancer is most often caused by exposure to tobacco smoke.

Fact 2: Tobacco is most commonly smoked in cigarettes.

Given the two facts above, answer the question "What is lung cancer most often caused by exposure to?" with the following options: 
- Chemical energy
 - carbon dioxide
 - asbestos
 - sunlight
 - water
 - fossil fuels
 - cigarettes
 - hydrocarbons


 

       cigarettes  