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 36 to 40

Coal, oil, and natural gas supply modern civilization (36)............. most of its power. However, not only are supplies of these fuels limited, but they are a major source of pollution. If the energy demands of the future are to be met without seriously harming the environment, existing (37)........energy sources must be improved or further explored and developed. These include nuclear, water, solar, wind, and geothermal power, as well as energy from new, (38)........... types of fuels. Each of these alternatives, however, has advantages and disadvantages.

Nuclear power plants efficiently produce large amounts of electricity without polluting the atmosphere; however, they are costly to build and maintain, and they pose the daunting problem of what to do with nuclear wastes. Hydroelectric power is (39)........ and environmentally safe, but impractical for communities located far from moving water. Harnessing energy from tides and waves has similar drawbacks. Solar power holds great promise for the future but methods of collecting and concentrating sunlight are as yet (40).........., as are methods of harnessing wind power.