You are presented with the question "what doesn't riding a bike cause?" and the following answer choices: 
- Allergies
 - harmful substances
 - Decrease bodily water
 - Greenhouse gasses
 - carbon monoxide
 - global warming
 - Dehydration
 - chemical changes

Now knowing that riding a bike does not cause pollution and carbon monoxide is, of course, an air pollutant, choose the best answer.


 

     carbon monoxide    