Fact 1: Carbon monoxide is a gas produced by furnaces and other devices that burn fuel.

Fact 2: Gas furnaces heat homes for up to 20 years.

Given the two facts above, answer the question "what produces carbon monoxide?" with the following options: 
- h2o
 - burning coal
 - hydrogen
 - freezing
 - heating homes
 - chlorofluorocarbons
 - cigarettes
 - gasoline


 

     heating homes    