Unit Twenty
　　Passage 2
　　Growth of trade will depend greatly on availability of energy sources. There may still be a trillion barrels of recoverable oil in the Middle East. But the oil crisis of 1974 has 11 to renewed interest in coal and to a search for 12 sources of energy. Solar, geothermal, and nuclear energy will play a large role in the years to come.
　　Solar energy is available in 13 forms. Buildings can be heated and cooled by direct use of solar radiation, crops and trees, which are the most efficient converters of sunlight into energy, can be grown for their energy potential, wastes can be burned as 14 , sunlight can be converted into DC (direct current) electricity, electric power can be 15 from the sun-warmed surface waters of the ocean, and lastly, solar radiation can be converted into heat that will drive electric power generators. Serious problems still remain as to 16 and storage of solar energy.
　　Geothermal energy is the energy contained within the earth. Heat is abundantly available deep in the earth’s core and is constantly being produced. However, this heat is usually located at too deep a level for 17 exploitation. In short, very little is known on the use of geothermal energy, and it has 18 been exploited.
　　Nuclear energy is produced in nuclear power plants. At these plants atoms of uranium are split, thus 19 masses of energy. Another source of energy
　　under development is the nuclear fusion of certain atoms of hydrogen. This could eventually 20 natural gas as a source of energy.

A. rarely	B.	transformation	C.	fuel	D.	replace
E. led	F.	alternative	G.	commercial	H.	briefly
I. derived	J.	various	K.	relieving	L.	releasing
M. transportation	N.	financial	O.	described

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