19. Nobody can predict exactly when supplies of fossil fuels will be __________ However, we all know they quickly reduce in quantity. A. installed B. polluted C. harmful D. exhausted 20. Biogas can be used to __________ natural gas in cooking, heating, or electrical generation. A. use up B. replace C. pollute D. install 21. At this time next week we__________ to work to support the air pollution cutting down campaign. A. are cycling B. will be cycling C. will cycle D. will be cycled 22. Someone’s carbon__________ is a measurement of the amount of carbon dioxide that their activities produce. A. footprint B. energy C. effect D. amount 23. Remember to__________ the lights before going to bed. A. turn on B. stop C. turn D. turn off 24. We will cut down in the use of natural gas because it is __________ and harmful to the environment. A. available B. abundant C. plenty D. limited 25. Hydro power is__________ because dams cannot be built in certain areas. A. abundant B. enough C. limited D. unlimited 26. __________ source of energy is the source that can’t be replaced after use. A. Effective B. Non-renewable C. Renewable D. Natural 27. Which of the following is NOT renewable source of energy? A. wind B. coal C. hydro D. solar 28. We are looking for cheap, clean and__________ sources of energy. A. serious B. dangerous C. effective D. efficient 29. By the middle of the 21st century, people in developing countries__________ more renewable energy. A. used B. have used C. uses D. will be using 30. If we go on__________ electricity, we will have to pay a lot next month. A. turning on B. widened C. wasting D. increasing 31. Biogas is__________ and cheap for cooking and heating. A. serious B. expensive C. plenty D. abundant 32. We should put__________ on our roof for the heating and hot water. A. equipment B. cracks C. solar energy D. solar panels 33. In many countries, people think that electricity, gas and water are not luxuries but__________. A. necessities B. appliances C. sales D. consumers