If buried vegetation in a wetland changes to peat over a long period of time, then buried vegetation in a wetland changes to peat over what?

Answer choices:
- a long period of time
 - The area was once underwater
 - a brief period of time
 - just a day's time
 - Physical weathering
 - It used to be covered by water
 - movements in the earth's crust
 - a quick period of time

 

 a long period of time         