Fact 1: Smoking causes direct damage to the lungs.

Fact 2: Mammals are warm blooded, use lungs for respiration, and have four legs.

Given the two facts above, answer the question "What harms the respiration system?" with the following options: 
- Pollution
 - sweating
 - Pesticides
 - exercise
 - smoking
 - parasites
 - swimming
 - breathing


 

     smoking    