You are presented with the question "What is not released after riding a bike?" and the following answer choices: 
- carbon monoxide
 - harmful substances
 - fossil fuels
 - heat energy
 - one celled organisms
 - hard outer covering
 - layers of fat
 - Pituitary gland

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        