Fact 1: Riding a bike does not cause pollution.

Fact 2: Carbon monoxide is, of course, an air pollutant.

Given the two facts above, answer the question "what doesn't riding a bike cause?" with the following options: 
- Allergies
 - harmful substances
 - Decrease bodily water
 - Greenhouse gasses
 - carbon monoxide
 - global warming
 - Dehydration
 - chemical changes


 

     carbon monoxide    