To produce energy from ______, a dam needs to be built across a powerful river.

• A. Biomass
• B. Water
• C. Solar
• D. Geothermal

Answer: (B)

The concept being tested is hydroelectric power, which uses water to generate electricity. When a dam creates a tall column of water behind its structure, gravity gives that water potential energy. As the water is released through the dam, it flows down through turbines, causing them to spin and drive generators to produce electricity. This setup specifically relies on manipulating water in a river to convert potential energy into electrical energy, which is why a dam is built across a powerful river.

Other energy sources listed don’t fit this mechanism: biomass uses organic material, solar relies on sunlight collected by panels or mirrors, and geothermal taps heat from underground. Hence, only water in a dam-driven system provides the necessary flow and head for hydroelectric generation.