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  3. We don’t see or hear them, but every day they quietly go about their work--filtering and cleansing our rivers and streams. And i

We don’t see or hear them, but every day they quietly go about their work--filtering and cleansing our rivers and streams. And i

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问题     We don’t see or hear them, but every day they quietly go about their work--filtering and cleansing our rivers and streams. And if we don’t act soon, they’ll disappear from the workforce just when we need them most. I am talking about pigtoes, monkeyface, pink heelsplitter and purple wartyback--freshwater mussels (贻贝) with funny names that belie the seriousness of their labors.  (41)  .
    One mussel alone can cleanse as much as a gallon of water per hour. Add up the work of a whole mussel community, and you get a virtual water treatment plant. According to Ethan Nedeau, an expert on the freshwater mussels of New England, even half the population of mussels at work in a one-half mile segment of New Hampshire’s Ashuelot River can help cleanse more than 11.2 million gallons of water a day--roughly the quantity of household water used by 112 000 people.
    (42)  . Today 69 percent of US freshwater mussel species are to some degree at risk of extinction or already extinct. The most diverse assemblage of freshwater mussels ever known was located in the middle stretch of the Tennessee River in northern Alabama. Before the damming of the river in the early 1900s, 69 mussel species had been spotted in this reach; 32 of them have apparently disappeared, with no recording sightings in nearly a century.
    (43)  . Like many freshwater mussels, the orange-nacre mucket has a fascinating life cycle and exhibits some of the most sophisticated mimicry in the animal kingdom. The females essentially use their offspring to lure fish into helping them colonize new stream bottoms. They package their larvae (幼虫) at the end of jelly--like tubes that can extend eight feet out into the water. To fish swimming by, the larvae dancing in the riffles of the river current looks like a tasty minnow. When the fish bites, the tube breaks, releasing the larvae into the stream. A few of the offspring attach to the fish’s gills and hitchhike around with their firmed host for a week or two, absorbing nutrients and growing along the way.  (44)  .
    Along with 16 other threatened or endangered mussel species in the Mobile watershed, the orange-nacre mucket is at risk of extinction--in large part due to excessive pollution and dams that have diminished the river habitat they need to survive. To me, the loss of such industrious, fascinating creatures diminishes more than our water quality-- it diminishes our natural heritage and our world.
    (45)  . So as we celebrate World Water Day, I hope we also celebrate the freshwater mussels that help keep our waters clean and healthy--and commit to efforts to conserve them.
    [A] My favorite freshwater mussel is the orange-nacre mucket, found only in the rivers and streams of Alabama’s Mobile River basin.
    [B] The United States ranks first in the world in the number of known species of freshwater mussels  292, com- pared with just 10 in all of Europe. But we’re losing these "living filters" all too fast.
    [C] Only habitat improvements, in some cases combined with mussel breeding and release efforts, can save these and the other 200 freshwater mussel species at risk nationwide.
    [D] Because I bet we’ll miss these little creatures with the whimsical names when they’re gone.
    [E] They suck water in, filter out bits of algae, bacteria and other tiny particles, and then release it back to the river cleaner than before.
    [F] Finally, the young mussels drop off, float to the river bottom, and colonize new territory--and before long begin their vital task of water purification.
    [G] It is our responsibility to take actions to protect the freshwater mussels, otherwise they will disappear in the future and the water will not be refreshed.  
选项
答案E
解析 文章开头一般阐述全篇的主旨或引出主题。本段引出了通篇要介绍的对象——淡水贻贝。通过第二段可知，下文介绍的是淡水贻贝净化水的作用，而作为文章的开篇并未提到淡水贻贝是如何净化水的，所以41题应当填入描写贻贝是如何净化水的句子，才使文章开头完整。选项巾[E]和[F]都提到净化水，但是作为文章开头，[F]中Finally是不恰当的，故[E]正确。  
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