Which statement about the carbon content of coal, oil, and natural gas is true?

• A. Coal has the lowest carbon per unit energy; oil is least versatile; natural gas is most carbon-intensive.
• B. Oil has the highest carbon per unit energy; Coal is least versatile; Natural gas is not used for heating.
• C. Coal has the highest carbon per unit energy; Oil has high energy density and is versatile; Natural gas has the lowest carbon per energy and is used for heating and electricity.
• D. Natural gas has the highest energy content per mass, surpassing coal.

Answer: (C)

The main idea here is carbon intensity per unit energy—how much carbon is released for the same amount of useful energy from each fuel. Coal tends to produce the most CO2 per unit of energy because it has a relatively high carbon content and, per kilogram, provides less energy than the lighter fuels. Oil is very energy-dense and versatile, so it also releases a lot of CO2 per energy, though generally less than coal. Natural gas, mainly methane, burns with comparatively less carbon per unit energy, so it has the lowest CO2 emissions per energy produced. It’s also widely used for heating and power generation because it burns cleanly and efficiently.

Other options miss the point by either mischaracterizing which fuel carries the most carbon per energy, or by focusing on energy density rather than carbon emissions per energy unit.