Which of the following is a source of sustainable energy?

• A. Coal
• B. Ethanol
• C. Oil
• D. Natural gas

Answer: (B)

Sustainable energy comes from sources that can be replenished on human timescales and generally produce lower environmental impact. Ethanol fits this idea as a biofuel produced from plant biomass such as corn or sugarcane. Plants absorb CO2 as they grow, and when ethanol is burned for energy, it releases carbon that was recently captured, creating a shorter, more sustainable carbon cycle compared with fossil fuels. If produced and managed responsibly, ethanol can be renewed each growing season and help reduce net greenhouse gas emissions relative to fossil fuels.

In contrast, coal, oil, and natural gas are fossil fuels formed over millions of years. They are finite and their combustion releases large amounts of carbon dioxide and other pollutants, contributing to climate change and air quality problems.