when did fossil fuels start being used
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when did fossil fuels start being used
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when did humans first use fossil fuels
when did fossil fuels become popular
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history of fossil fuels
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origins of fossil fuels
when did fossil fuels start being used, when did fossil fuels form, when did humans first use fossil fuels, when did fossil fuels become popular, when did humans start using fossil fuels, when were fossil fuels created, when was fossil fuels invented, history of fossil fuels, how long ago were fossil fuels created, origins of fossil fuels