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Most fuels we use are remains of living beings, mostly plants, that lived millions of years ago. That is why they are called FOSSIL FUELS. Oil, natural gas and coal are fossil fuels. In the earth there are deposits of coal and oil that, on many occasions, contain big bags of natural gas (called associated gas). But, sometimes, the gas got away through a crack, so it is possible to find isolated bags of natural gas without any oil or coal. When they burn they sed carbon dioxide and other polluting gases to the atmsphere.
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