Answer :
how fossil fuels were formed?
What are fossil fuels?
It is a natural fuel that is formed of hydrocarbons. It includes coal or natural gas.
How are fossil fuels formed?
Prehistoric plants and animals died and were buried. Due to the conditions like high pressure and temperature their decomposition occurs and resulted in the formation of different kinds of fuels.
Hence option a) Trees were cut down and heated is the most appropriate.
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