prehistoric means something that existed a very long time ago0 & plants are a kind of vegetation ->  forming fossil fuels requires deposition and burial of decaying plants  the formation of fossil fuels is a kind of process & if something is required for a process then that something helps in that process ->  if something is required for the formation of fossil fuels then that something helps in the formation of fossil fuels  forming fossil fuels requires deposition and burial of decaying plants & if something is required for the formation of fossil fuels then that something helps in the formation of fossil fuels ->  decaying plants help in the formation of fossil fuels  if something is formed from something else over a very long period of time, then that something else existed a very long time ago & fossil fuels / fossils forming occurs over a period of 300000000 years which is considered a very long time to a human ->  the organisms that formed fossil fuels existed a very long time ago  the organisms that formed fossil fuels existed a very long time ago & prehistoric means something that existed a very long time ago ->  the organisms that formed fossil fuels are prehistoric  the organisms that formed fossil fuels are prehistoric & prehistoric means something that existed a very long time ago1 ->  the plants that formed fossil fuels are prehistoric  decaying plants help in the formation of fossil fuels & the plants that formed fossil fuels are prehistoric ->  prehistoric decaying plants help in the formation of fossil fuels  oil is a kind of fossil fuel & coal is a kind of fossil fuel ->  coal and oil are kinds of fossil fuels  prehistoric means something that existed a very long time ago2 & natural gas is a kind of fossil fuel ->  gas is a kind of fossil fuel  coal and oil are kinds of fossil fuels & gas is a kind of fossil fuel -> forming fossil fuels requires deposition and burial of decaying plants0: coal and oil and gas are kinds of fossil fuels  forming fossil fuels requires deposition and burial of decaying plants0 & prehistoric decaying plants help in the formation of fossil fuels -> prehistoric decaying plants help in the formation of coal and oil and gas