Read the text. You have to choose the best answer: A, B, C, or D. Then on your answer sheet, find the number on the question and mark your answer.
 

A power station (also referred to as a generating station, power plant powerhouse or generating plant) is an industrial facility for the purpose of generating electric power. Most power stations contain one or more generators, a rotating machine that converts mechanical power into electric power. The relative motion between a magnetic field and a conductor creates an electrical current. The energy source harnessed to tum the generator varies widely. Most power stations in the world burn fossil fuels such as coal, oil and natural gas to generate electricity. Others use nuclear power but there is an increasing use of cleaner renewable sources such as solar, wind, wave, and hydroelectric.

Adapted from Wikipedia.org

Which one is NOT used as the energy source in a power station?