Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 18 to 24.

In the twentieth century, people demand on unlimited energy to power their everyday lives. A wide range of energy-run devices and modern conveniences are taken for granted, and although it may seem that we will never be in danger of living without those conveniences, the fact is that many supplies of energy are dwindling rapidly. Scientists are constantly searching for new sources of power to keep modern society running. Whether future populations will continue to enjoy the benefits of abundant energy will depend on the success of this search.

Coal, oil and natural gas supply modern civilization with 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 alternative 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, non-polluting 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 inexpensive 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 inefficient, as are methods of harnessing wind power.

Every source of energy has its disadvantages. One way to minimize them is to use less energy. Conservation efforts coupled with renewable energy resources, such as a combination of solar, water, wind and geothermal energy and alternative fuels, such as alcohol and hydrogen,, will ensure supplies of clean, affordable energy for humanity’s future.

What does the word its in paragraph 2 refer to? A. modern civilization B. coal C. oil D. natural gas