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. Given these facts, what is compressed into coal over long periods of time among the following options:
- an object
 - bottom of feet
 - energy
 - carbon dioxide
 - rays or beams
 - kinetic energy
 - electricity
 - vegetation


 

        vegetation 