Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word for each of the blanks.

The search for alternative sources of energy has led in various directions. Many communities are burning garbage and other biological waste products to produce electricity. Converting waste products to gases or oil is also an efficient way to dispose of wastes.

Experimental work is being done to derive synthetic fuels from coal, oil shale, and coal tars. But to date, that process has proven expensive. Other experiments are underway to harness power with giant windmills. Geothermal power, heat from the earth, is also being tested.

Some experts expect utility companies to revive hydroelectric power derived from streams and rivers. Fifty years ago, hydroelectric power provided one third of the electricity used in the United States, but today it supplies only four percent. The oceans are another potential source of energy. Scientists are studying ways to convert the energy of ocean currents, tides, and waves to electricity. Experiments are also underway to make use of temperature differences in ocean water to produce energy.

What is the passage mainly about?