Arrange the following fuels in decreasing order in terms of the amount of carbon dioxide emitted per unit of energy output from their combustion. Choose the correct answer from the options given below:

In decreasing order of carbon dioxide emitted per unit of energy the sequence is coal, then gasoline, then propane, then natural gas.

Coal, here lignite, releases the most carbon dioxide per unit of energy because of its high carbon and low hydrogen content.

Gasoline comes next, emitting less than coal but more than the lighter gaseous fuels.

Propane emits less still, as its molecules carry more hydrogen relative to carbon.

Natural gas, mostly methane, releases the least carbon dioxide per unit of useful energy.

This is why switching from coal to gas is often promoted as a way to cut emissions.