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  3. Scientists have devised a way to determine roughly where a person has lived using a strand(缕)of hair, a technique that could hel

Scientists have devised a way to determine roughly where a person has lived using a strand(缕)of hair, a technique that could hel

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问题     Scientists have devised a way to determine roughly where a person has lived using a strand(缕)of hair, a technique that could help track the movements of criminal suspects or unidentified murder victims.
    The method relies on measuring how chemical variations in drinking water show up in people’s hair.
    " You’re what you eat and drink, and that’s recorded in your hair," said Thure Cerling, a geologist at the University of Utah.
    While U. S diet is relatively identical, water supplies vary. The differences result from weather patterns. The chemical composition of rainfall changes slightly as rain clouds move.
    Most hydrogen and oxygen atoms in water are stable, but traces of both elements are also present as heavier isotopes(同位素). The heaviest raid falls first. As a result, storms that form over the Pacific deliver heavier water to California than to Utah.
    Similar patterns exist throughout the U. S. . By measuring the proportion of heavier hydrogen and oxygen isotopes along a strand of hair, scientists can construct a geographic timeline. Each inch of hair corresponds to about two months.
    Cerling’s team collected tap water samples from 600 cities and constructed a map of the regional differences. They checked the accuracy of the map by testing 200 hair samples collected from 65 barber shops.
    They were able to accurately place the hair samples in broad regions roughly corresponding to the movement of raid systems.
    "It’s not good for pinpointing(精确定位)," Cerling said. "It’s good for eliminating many possibilities. "
    Todd Park, a local detective, said the method has helped him learn more about an unidentified woman whose skeleton was found near Great Salt Lake.
    The woman was 5 feet tall. Police recovered 26 bones, a T-shirt and several strands of hair.
    When Park heard about the research, he gave the hair samples to the researchers. Chemical testing showed that over the two years before her death, she moved about every two months.
    She stayed in the Northwest, although the test could not be more specific than somewhere between eastern Oregon and western Wyoming.
    "It’s still a substantial area," Park said "But it narrows it way down for me. "
What is said about the rainfall in America’s West?
选项 A、There is much more rainfall in California than in Utah.
B、The water it delivers becomes lighter when it moves inland.
C、Its chemical composition is less stable than in other areas.
D、It gathers more light isotopes as it moves eastward.
答案B
解析 定位在原文第五段。这段范围的意思大致是“氢和氧这两种原子有同位素(同位素对于学理的同学要好理解一些，就是同是氢氧原子，有的原子量大一些，水就重一些，原子量小一些，水就轻一些，但这点原文并没有解释，所以加大了这里理解的难度)，最重的雨会首先降下来，结果导致了在太平洋洋面上形成的雨会在离海岸较近的加利福尼亚州先降下来，所以选B。(此时的雨比较重，是含大原子量同位素的水，而轻一点的雨会等到更内陆的地方比如犹他州再降下来。)
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