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

買知識，賣知識，就上233考試大！還可以免費賺取現(xiàn)金哦！
·您手頭有好東西嗎？上傳上來和大家一起分享吧！你也可以求購你自己想要的資料。 　　·上傳您的精品資料，可獲得收入，既可在網站上進行消費，也可提取現(xiàn)金。 　　·若被評為資料，還可以增加任務分值。了解更多>>

　　編輯推薦：
　　2012年12月大學英語四級報名時間
　　2012年12月英語四級考試時間及流程