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

Fact 2: Fuel is burned inside the engine.

Given the two facts above, answer the question "Carbon monoxide, a product of furnaces and engines, is a:" with the following options: 
- acid
 - CO2
 - kill
 - harm
 - gas
 - form
 - CO 2
 - HPV


 

     gas    