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 is not released after riding a bike?" with the following options: 
- carbon monoxide
 - harmful substances
 - fossil fuels
 - heat energy
 - one celled organisms
 - hard outer covering
 - layers of fat
 - Pituitary gland


 

 carbon monoxide        