A
The fact that trees are vital to our life is not a secret. They provide us with food, wood and most importantly, oxygen. Now there is one more thing we can add to this list — blocking out harmful bacteria from water.
The discovery was made by a team consisting of scientists from the Massachusetts Institute of Technology (MIT) and high school students who were seeking a natural water filter (过滤器) — one that would help communities in developing countries that do not have access to modern water filter systems.
The researchers, led by Professor Rohit Karnik, decided to turn to trees for help because they could allow liquid to flow through, while blocking out air bubbles.
They began by cutting 1.5inchwide 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 of different sizes through. To their surprise, 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 held back 99% of the bacteria, allowing only 1% to flow through.
Mr. Karnik said that the bark was able to filter water containing much smaller particles. This means that they 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 bark isn’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 damp. This means that before this natural filter can be put into 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 ability 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 (气孔). They think that these may be more effective in trapping smaller particles and even viruses.
语篇解读：最近科学家们发现：树皮可以过滤水中的细菌。这一发现可以改善发展中国家人们的生活质量。
21．What problem troubles the researchers when they use tree bark as water filters?
A．Viruses cannot be blocked out.
B．Not all bacteria will be held back.
C．Viruses don’t make air bubbles.
D．Tree bark needs to be kept in water.
解析：选A 细节理解题。从文章第五段末的“However, what troubles them is that tree bark isn’t blocking out viruses, which tend to be much smaller.”可知，让研究人员感到困扰的是树皮无法挡住病毒，因为病毒要比细菌小得多，故选A项。
22．We can learn from the text that tree bark can filter water when ________.
A．it is damp
B．it is dry
C．it has been cut down
D．it is processed by researchers
解析：选A 细节理解题。从文章倒数第二段中的“it only works when damp”和“the researchers will have to figure out how to keep the wood damp”可知，树皮只有在潮湿的时候才有过滤作用，故选A项。
23．The team is planning to conduct other experiments to ________.
A．find all wood bark with such ability
B．find which trees have the smallest pores
C．improve trees’ filtering ability
D．test other wood bark’s filtering ability
解析：选D 推理判断题。从文章最后一段的内容可知，研究者们打算用其他种类的树皮进行实验，目的是测试其他种类树皮的过滤能力，故选D项。
24．In which column of a newspaper can we read this text?
A．Nature. B．Science.
C．Lifestyle. D．Society.
解析：选B 文章出处题。本文是一项有关科研方面的发现，因此最有可能出现在报纸的科学专栏中，故选B项。
