The search for alternative sources of energy has led in various directions. Many communities are burning garbage (1)______ to produce electricity. Converting waste products to gases or oil is also an efficient way to dispose of waste. Experimental work is being done to derive synthetic fuels (2)_____ . But to date, the process has proven expensive. Other experiments are underway to harness power with giant windmills. Geothermal power, heart form earth, (3)_____ .

Some experts expect utility companies to revive hydroelectric power derived form streams and rivers. Fifty years ago hydroelectric power provided one third of the electricity used in the United States, (4)_______ . 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 the temperature differences in ocean water to produce energy.

A. and other biological waste products

B. from coal, oil shale and coal tats

C. is also being tested

D. but today it supplies only four percent

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