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  3. 共用题干 The Exploding Lakes of CameroonWhat comes to mind when you think of a l...

共用题干 The Exploding Lakes of CameroonWhat comes to mind when you think of a l...

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问题 共用题干
The Exploding Lakes of Cameroon
What comes to mind when you think of a lake?You probably imagine a pretty scene with blue water, birds,and fish.For the people in the northwestern Cameroon,however,the image is very different.For them, lakes may mean terrible disasters.In 1984,poisonous gases exploded out of Lake Monoun and came down into the nearby villages,killing thirty-seven people.Two years later,Lake Nyos erupted.A cloud of gases rolled down the hills and into the valleys and killed 1,700 people.
Lake Nyos and Lake Monoun are crater（火山口）lakes.They were formed when water collected in the craters of old volcanoes.The volcanoes under Lake Nyos and Lake Monoun are not active anymore.However, poisonous gases from the center of the earth continue to flow up through cracks in the bottom of the lake.This is normal in a crater lake.In most crater lakes,these gases are released often because the water"turns over" regularly.That is,the water from the bottom of the lake rises and mixes with the water at the top,allowing the gases to escape slowly.
However,in Lakes Nyos and Monoun,there is no regular turning over.No one knows the reason for this fact,but as a result,these lakes have more gases trapped at the bottom than other crater lakes.In fact, scientists who have studied Lakes Nyos and Monoun have found 16,000 times more gases.When a strong wind,cool weather,a storm,or a landslide（滑坡）causes the water to turn over suddenly,the gases escape in a violent explosion.
In the past,no one knew when the gases might explode,so there was no way for the villagers to escape disaster.Now scientists from the United States,France,and Cameroon have found a way to reduce the gas pressure at the bottom of Lake Nyos.They stood a 672-foot plastic pipe in the middle of the lake,with one end of the pipe near the bottom and the other end in the air.Near the top of the pipe,the team put several holes that could be opened or closed by a computer.Now,when the gas pressure gets too high,the holes are opened and some of the gas-filled water shoots up through the pipe into the air like a fountain.With less pressure,a disastrous explosion is much less likely.However,the scientists are not sure that one pipe will be enough to prevent explosions.They hope to put in others soon and they plan to install a similar pipe and a computer system at Lake Monoun as well.
To protect people nearby until all of the pipes are in place,the scientists have installed early warning systems at both lakes.If the gas pressure rises to a dangerous level,computers will set off loud sirens（警 报）and bright lights to warn the people in the villages.In that way,they will have time to escape from the dangerous gases.
选项 Lake Nyos and Lake Monoun explode because_____.
A:the gases rise to the top and mix with air
B:people from the villages turn over the water
C:scientists have put in a computer system
D:they have more gases trapped at the bottom than other crater lakes
答案D
解析第一段第五、六、七句提到，Monoun湖在1984年喷发出了有毒气体，两年后，Nyos湖喷发时也出现类似情况，因此选择B项。
根据第二段倒数第二句可知，大多数火山口湖中的湖水会翻转，使有毒气体得以释放，而第三段第一句又表示，Nyos和Monoun中的湖水不会翻转。由此可知，Nyos和Monoun与其他的火山口湖不同。
根据第三段第二句可知，虽然没人知道原因，但是与其他火山口湖相比，Nyos和Monoun的湖底释放出了更多的气体。
根据第四段第三句“They stood a 672-foot plastic pipe in the middle of the lake , with oneend of the pipe near the bottom and the other end in the air.”可知，该科学家团队在湖里立了一根管子，管子的一端在湖底，另一端在空中。这样做是在利用连通器的原理来减小湖底压强，因此A项正确。
前文表明，湖里竖起若干根减小湖底压强的管子后，就不会再有毒气喷发的危险。最后一段又说在这些管子都到位前科学家在这两个湖附近安装了早期预警系统，以便人们有时间逃离这些有毒气体.由此可知，这种有毒气体喷发的灾难在未来可以避免。
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