23. A. will be B. are C. have been D. has been
II. Read the passage carefully and choose the best answer.
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 is also being tested. Some experts expect utility companies to revive hydroelectric power derived
from streams and rivers. Fifty years ago hydroelectric power provided about 33 percent of the electricity used in the
United States, but today it supplies only 4 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.
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