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We need energy to live and work. Our major source of (19) _________ is oil. Oil is one kind of fossil fuel. The amount of fossil fuels in the world is (20) _________. Therefore, we must save it, and at the same time, we must find new sources of energy. Geothermal heat and nuclear power are alternative (21) _________ of energy. They can give us electricity. Other alternative sources (22) _________the sun, waves and water. These sources are not only unlimited and available (23) _________ clean and safe for the environment. People should develop and use them more and more in the future.