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  3. 　　A river’s colors hold clues to what flows in its water, from soft-green algae to yellow-brown mud. Human eyes might miss【B1】__

　　A river’s colors hold clues to what flows in its water, from soft-green algae to yellow-brown mud. Human eyes might miss【B1】__

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问题 　　A river’s colors hold clues to what flows in its water, from soft-green algae to yellow-brown mud. Human eyes might miss【B1】________shifts in these shadings, but satellites can【B2】________them—and researchers can use them to track large-scale changes and【B3】________spot【B4】________of trouble.
　　A team led by University of Pittsburgh environmental scientist John R. Gardner analyzed 234,727 satellite images,【B5】________67,000 miles of U. S. rivers over 35 years. The researchers isolated the main light wavelength each river reflected,【B6】________ shifting colors into grids of numbers.
　　Gardner’s group found sediment-laden yellow (56 percent) and algal green (38 percent)【B7】________the country’s rivers. A third of them had changed color over the past few decades,【B8】________the fastest shifts often near sediment-trapping dams. This effect is clear and【B9】________
　　Western U.S. river colors【B10】________moved toward the blue end of the spectrum over time, suggesting they carried【B11】________sand and silt. But many rivers in the Northeast showed a "red shift" that made them appear more yellow, indicating lower water levels or【B12】________sediment. Gardner says this【B13】________suggests that regional factors,  such as land-use patterns and watershed-management【B14】________, influence long-term river hue shifts.
　　"We also found very distinct【B15】________patterns," Gardner says. Many of the nation’s rivers turn yellower in spring or summer【B16】________peaking rainfall muddies the waters.【B17】________the timing also【B18】________geography and human activity such as【B19】________
　　Using satellites to gauge changes in river composition could warn scientists when environments begin to【B20】________balance, the researchers say.
【B16】
选项 A、if
B、while
C、though
D、as
答案[D]
解析 本题考查连词辨析。四个选项分别为[A]if“如果”；[B]while“虽然，然而，当……时”；[C]though“虽然”；[D]as“因为，由于”。该空前后两个句子意思之间的逻辑关系是因果关系，空前为果，空后为因。as作连词意为“由于，因为”。故选项[D]正确。
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