alternative polluted harmful expensive

We need energy to live and work. Our major source of…(36)……….is oil. Oil is one kind of fossil fuel. The amount of fossil fuel in the world is……(37)….Therefore, we must save it, and the same time, we must find new sources of energy. Geothermal heat and nuclear power are……(38)….sources of energy. They can give us electricity. Other alternative sources are the sun, waves and water. The sources are not only …(39)……and available but also clean and safe for the…(40)…People should develop and use them more and more in the future.

Question 38: