Fact 1: Being burried under soil and mud changes peat into coal through extreme heat and pressure in a swamp over a long period of time.

Fact 2: Swamp vegetation was compressed to form coal.

Given the two facts above, answer the question "What is compressed into coal over long periods of time?" with the following options: 
- an object
 - bottom of feet
 - energy
 - carbon dioxide
 - rays or beams
 - kinetic energy
 - electricity
 - vegetation


 

        vegetation 