Concept:Carbon monoxide (
CO) is a poisonous gas produced mainly by incomplete burning of fuel in places with many cars.
Explanation:In cities, motor vehicles are the largest source of
CO.
When petrol or diesel does not burn completely inside an engine, it releases
CO instead of carbon dioxide.
This happens because the oxygen supply is limited during combustion in traffic.
Other sources like forest fires and industrial processes can also produce
CO, but they are less constant in urban areas.
Vehicle emissions are therefore the main urban source of carbon monoxide.
Answer:Vehicle emissions.