Background Paragraph: Gasoline-powered vehicles emit a lot of harmful materials. Nitrogen oxides are formed when atmospheric nitrogen reacts with oxygen at the high temperatures found in a car engine. Carbon monoxide is a by-product of incomplete combustion of hydrocarbons. Evaporated and unused fuel releases volatile hydrocarbons into the atmosphere to help form smog. The presence of a catalytic converter in the exhaust system of the car causes these materials to react and be changed into less harmful products. 
Story: Bend village was located 20 miles away from Mound village. People in Bend village used gasoline powered vehicles, while Mound village used only electrical vehicles. 
Question: Which village emitted less carbon monoxide?
Mound.