Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 17 to 21.

We need energy to live and work. Our major source of (17) _________is oil. Oil is one kind of fossil fuel. The amount of fossil fuels in the world is (18) _________. Therefore, we must save it, and at the same time, we must find new sources of energy. Geothermal heat and nuclear power are alternative (19) _________of energy. They can give us electricity. Other alternative sources (20) _________ the sun, waves and water. These sources are not only unlimited and available (21)_________clean and safe for the environment. People should develop and use them more and more in the future.

We need energy to live and work. Our major source of (17) _________is oil.