Fact 1: Air pollution harms the respiratory and circulatory systems.

Fact 2: Smoking is a form of air pollution.

Given the two facts above, answer the question "What harms the respiratory and circulatory systems?" with the following options: 
- Cars
 - oxygen
 - smoking
 - vehicles
 - emissions
 - gasoline
 - fresh air
 - lungs


 

   smoking      