【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.Soppose that a mine is established on a lode deposit that contains ore with 20 times the average proportion of gold to ore.Which one of tha following describes the number of days it would take for such an operation, working at the fastest rate described in the passages, to produce 100 metric tons of pure gold? A. Greater than 1 but less than 2 days. B. Greater than 2 but less than 3 days. C. Greater than 3 but less than 4 days. D. Greater than 4 but less than 5 days. E. Greater than 5 but less than 6 days.
请问一下为什么选D，怎么理解？ 感激不尽! | 
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发表于 2014-8-6 22:47:37
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