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

Fact 2: Cars are a significant source of air pollution.

Given the two facts above, answer the question "What harms the respiratory and circulatory systems?" with the following options: 
- oil
 - rain
 - sulfur
 - cars
 - tires
 - coal
 - tar
 - bicycles


 

    cars     