During the earlier times of the Earth plants and animals 1) had lived/ lived/were living on the surface until they eventually died and decomposed. They 2) would accumulate/ were accumulating/accumulating on the bottom of the global seabed. We know that those plants and creatures eventually 3) transformed/ transforming/ been transformed under heat and pressure into fossil fuels. They 4) rot/ had rotten/ were rotting and 5) were moldering/had moldered/ moldering while temperature and pressure 6) was increasing/increased/were increasing. After a very long period of time, mineral cover 7) had formed/ is formed/forms on top of this mass and it began 8) squeeze/ being squeezing/Squeezing oil and gas more and more. Rocks 9) have trapped/were trapping/ had trapped gas and oil in their pores and this is how oil and gas reservoirs 10) took/ have taken/ had been taking shape under the earth. This treasure 11) lie/ had been lying/ would lie deep beneath the surface for quite a long time. Since prehistoric times, these underground reservoirs with fuel 12) had held/ holded/ has held tremendous amounts of fossil fuels until the time came to use them. Only two natural elements such as hydrogen and carbon 13) mix/mixing/are mixed together 14) create/ creating/to create fossil fuels till the end of that process. The fuel 15) had been storing/ stored/ had stored more and more energy until people discovered it. It's original strength 16) would come/ is coming/came from the sun and 17) has changed/ changed /was changing o natural burning substance. Millions of years 18) had passed/ were passing/had been passed and people 19) were not noticing/had not noticed/were not noticed the sources of energy inside the earth till the discovery of fire. Fossil fuel suddenly 20) had become /would become /became necessary.

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