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

Fact 2: Air pollution, or smog, is caused by cars and trucks.

Given the two facts above, answer the question "What harms the respiratory system?" with the following options: 
- coal
 - oxygen
 - tar
 - cigars
 - breathing
 - silence
 - sulfur
 - cars


 

        cars 