You are presented with the quiz "What is compressed into coal over long periods of time?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "being burried under soil and mud changes peat into coal through extreme heat and pressure in a swamp over a long period of time" and "swamp vegetation was compressed to form coal". 

So, what's the best answer to the question?


 

        vegetation   