Read the passage carefully, then choose the correct answers. 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 ...

Read the passage carefully, then choose the correct answers.

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 one third of the electricity used in the United States, but to day 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.

According to the author, the impracticability of using coal, oil shale and coal tars as sources of energy is due to ___________ .