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【Prospector】 Gold is typically minedfrom two different types of geologic formations (known as deposits): lode depositsand placer deposits. Lode deposits are what prospectors dream of finding: largedeposits of nearly pure gold. Such deposits are located where they wereoriginally deposited by the mineral-bearing solutions that carried the gold upfrom the earth’s interior. Placer deposits, on the other hand, come from preexistinglode deposits’ exposure to the weather causes gold to be released from the surroundingrock and transported by rivers in the form of dust or flakes. When a streamcarrying the gold slows, thegold collects in pockets of sand. Placer-deposit mines have historically beenthe source of approximately 35 percent of the total goal mined in the US. However,in recent years, the quantity of gold mined from such deposits has decreased asthe readily accessible deposits have been exhausted. Thus, despite an increasein net gold mined, placer-deposit mining now accounts for only a few percent oftotal gold mined in the US. 【Environmentalscientists】 Because—unlike mining lodedeposits—mining placer deposits does not usually involve crushing rock andusing chemicals to extract gold, the environmental impacts are generally lessthan those of mining lode-deposits. The primary impacts of placer-depositmining are habitatdestruction and sediment release. Habitat destruction occurs as a result ofriver diversions and disruptions of river bottoms and banks, and the largeamounts of silt and sediment released can severely impact water quality. Moderncommercial operations tend to use setting ponds to prevent this discharge. Mining lodedeposits has a much larger environmental impact by virtue of size of suchoperations, the generation of waste material, and the use of toxic chemicals. Thegold comes out of the ground as raw ore- gold aggregated with other minerals. Onaverage, such mining operations process approximately 130 kilograms of raw oreto produce 1 gram of pure gold. Unlike placer-deposit mines, modern commerciallode-deposit mines are massive operations, some displacing and processing up to180,000 metric tons (1 metric ton = 1,000 kilograms) of raw ore per day
PREP 14-4. For which of the followingquestions concerning the time the passages were written does the informationprovided supply the clearest answer? A. Howmuch raw ore did the average lode-deposit mine process daily? B. Whatwas the total annual quantity of pure gold produced globally by placer-depositmines? C. Whatwere the principal forms of environmental damage attributable to chemicals usedto extract gold from raw ore? D. Whatwere the principal forms of environmental damage attributable to mining placerdeposits? E. Whatwas the total annual quantity of gold mined in the USA? 请问一下题目的意思是什么好吗？怎么理解？为什么选D，感激不尽!
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发表于 2013-6-15 23:50:20
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