What are the four "stages" of coal formation? How do the four types of coal differ?
a. Peat - Lignite - Bituminous - Anthracite
b. Peat - Anthracite - Bituminous - Lignite
c. Lignite - Peat - Anthracite - Bituminous
d. Anthracite - Bituminous - Lignite - Peat