Carbon monoxide is a source of ____ pollution.

Carbon monoxide is a poisonous gas released into the air. It comes from car exhaust, burning fuels, and incomplete combustion of materials.

Since carbon monoxide is a gas, it spreads through the atmosphere, not in land or water. It mixes with air and can be breathed in by people and animals.

Carbon monoxide specifically pollutes the air we breathe. It does not directly pollute land or sea, making air pollution the correct answer.