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: Fens form non-acidic peat.

Given the two facts above, answer the question "being burried under soil and mud changes how what forms?" with the following options: 
- cacti
 - deltas
 - fens
 - potash
 - bamboo
 - rivers
 - roots
 - mildew


 

   fens      