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fossil fuels are formed from the buried remains of plants and animals over a period of millions of years.coal ,petroleum and natural gas are its examples.

we should conserve them because it takes millions of years to form fossil fuels.if we run out of the existing stock of these fuels,we will not get more until natural processes have regenerated them.

i hope it helps....
fossils fuel are formed under very high pressure ,when plants animals decay there dead bodies mixes up with the soil and then it goes under many layers of soil and after millions of years under a very high pressure it transforms into petroleum and many other. since they are formed from fossils therefore they are called as fossils fuels . 
we should conserve them because they need millions of year to form and many people use these fuels as an energy source.however, fossil fuels are non-renewable if resources are once vanished then they will never be available again. It is therefore important to conserve fossil fuels, using alternative sources of energy when possible.

              i hope it helps.............................^_^

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