Final answer:
Coal is classified into four main types, including anthracite, bituminous, subbituminous, and lignite. As coal transitions from anthracite to bituminous to lignite, the energy output decreases and more coal needs to be mined to achieve the same energy output in terms of mass removed.
Step-by-step explanation:
Coal is a complex solid material derived primarily from plants that died and were buried hundreds of millions of years ago and were subsequently subjected to high temperatures and pressures. It is a combustible black or brownish-black sedimentary rock with a high amount of carbon and hydrocarbons. Coal is classified into four main types, or ranks, including anthracite, bituminous, subbituminous, and lignite. Anthracite coal is the highest grade of coal and has the lowest hydrogen:carbon ratio. As coal transitions from anthracite to bituminous to lignite, the energy output decreases and more coal needs to be mined to achieve the same energy output in terms of mass removed.