Thefactthattreesare vital toourlifeisnosecret.Theyprovi...

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The fact that trees are vital to our life is no secret. They provide us with food, wood and most importantly, oxygen. Now there is one more thing you can add to this list -blocking out harmful bacteria from water.

    The discovery was pointed out by a team made up of scientists from the Massachusetts Institute of Technology (MIT) and high school student who were seeking a natural water filter — one that would help communities in developing countries that are not equipped with modern water systems.

    The researchers, led by Rohit Karnik, a professor at MIT, decided to turn to trees for help because they allow liquid to flow through, while blocking out air bubbles.

    They began by cutting 1.5-inch-wide sections of tree bark from the branches of a white pine tree. The researchers then tested the wood's filtering ability by pouring water containing red dye particles (粒子) of different sizes, through. To their astonishment, they found that it was effective in trapping all the particles. Encouraged, the team conducted another experiment, this time with water that contained bacteria. Sure enough, the sapwood (邊材) held back 99% of the bacteria, allowing only 1% to flow through.

   Mr. Karnik says that the bark was able to filter water containing much smaller particles. This means that barks can be used to get rid of a majority of bacteria, since most are at least 10 times as wide. However, what troubles them is that tree barks aren’t blocking out viruses, which tend to be much smaller.

    Also, while the pine sapwood is effective, we must remember one thing—it only works when it is moist. This means that before this natural filter can be put to practical use, the researchers will have to figure out how to keep the wood damp, or find a way to keep the bark's great filtering capability even when it's dry!

   But the team is not done yet. They plan on testing other types of sapwood, including those from flowering trees which have smaller pores (氣孔). They think that these may be more effective in trapping smaller particles and even viruses

32. How could the scientists’ discovery about trees affect some developing countries?

  A. More pine trees will be planted.               B. Bacteria will be removed from water.

  C. People won’t worry about water pollution.       D. Clean water will become more accessible.

33. In the experiments, the researchers ________.

  A. found tree barks can’t get rid of viruses          B. used water containing particles of similar size

  C. were surprised tree barks remove all the bacteria   D. made sure most particles were bigger than bacteria

24. The underlined word “moist” in the passage probably means ________.

  A. hot                                       B. cold

  C. dry                                       D. wet

35. The team is going to conduct other experiments to ________.

  A. search for more kinds of wood barks             B. find a way to keep the wood damp

  C. improve the filtering ability of trees              D. test the filtering ability of other trees

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