premise: if something formed from something else over a very long period of time then that something else existed a very long time ago sent2: prehistoric means a very long time ago sent3: if a living thing dies then that living thing is dead sent4: fossil fuels / fossils forming occurs over a period of 300000000 years which is considered a very long time to a human sent5: oil is a kind of fossil fuel sent6: a living thing is an organism sent7: an animal is a kind of organism sent8: dead organisms decay sent9: fossil fuels / fossils are formed from dead organisms over time.
hypothesis: oil is formed from prehistoric animals dying and decaying