Fact 1: Using a gasoline engine requires burning gasoline.

Fact 2: And, of course, burning of gasoline produces toxic carbon monoxide.

Given the two facts above, answer the question "Toxic carbon monoxide is produced by what?" with the following options: 
- gasoline engines
 - Electric generators
 - electric cars
 - electricity
 - solar panels
 - cigarettes
 - an engine
 - ionization


 

 gasoline engines        