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  3. Scientists have begun using satellite-based systems to predict volcanic eruptions. This allows blanket coverage of the entire wo

Scientists have begun using satellite-based systems to predict volcanic eruptions. This allows blanket coverage of the entire wo

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问题     Scientists have begun using satellite-based systems to predict volcanic eruptions. This allows blanket coverage of the entire world. Envisat is the most advanced craft of this type. It has sensors that, by recognizing the characteristic optical signature of various substances in the atmosphere, can detect the presence of specific gases, such as sulphur dioxide. These may indicate an imminent volcanic eruption.
    Mike Abrams, a scientist at America’s space agency, NASA, says that the limiting factor on all such satellites is, surprisingly, not technological, but temporal. The orbits in which they are placed allow them to observe the entire earth, but they cannot pass over a particular spot more frequently than once every 15 days or so. Dr. Abrams argues that more satellites are needed to gather sufficient data on the earth’s volcanoes. Existing satellites, though, do at least serve as an early warning system. If one of them detects something amiss, local volcanologists can then intensify their local monitoring efforts. If necessary, they can even initiate an evacuation.
    In addition, it is important to predict lahars, flowing mixtures of rock, debris, ash and water, which are often set off when hot lava meets cold snow. Lahars can travel down a mountain as fast as 60 kilometres per hour. In 1985 a lahar set off by the eruption of the Nevada del Ruiz volcano in Colombia killed 25,000 people. With acoustic flow monitors, such as those developed by the USGS in the last five years, such a tragic loss of life might have been averted. The flow monitors are seismometers, which measure vibration, and are sensitive to higher frequencies than those used to record earthquakes and volcanic activity. This lets them hear lahars from a long way off, creating a valuable extra hour of time that can be used to evacuate. Such systems are now in place all round the world.
    Volcanic eruptions are dangerous not only to people on the ground, but also to those in the air. According to the USGS, more than 80 commercial aircraft have run into unexpected volcanic ash in the past 15 years, with the resulting damage costing hundreds of millions of dollars. If the optimists are right, the threat from volcanoes could one day become a known one. There is a problem, though. The work that needs to be done relies on the continual use of satellites and ground monitoring. Volcanologists are now issuing a new warning: that it is particularly hard to get funding for this kind of work. This is something that everybody should be bothered about.
The example of a lahar from the Nevada del Ruiz volcano in 1985 is used to show that
选项 A、acoustic flow monitors are effective in predicting lahars.
B、acoustic flow monitors are used all around the world.
C、acoustic flow monitors can completely prevent lahars.
D、acoustic flow monitors are sensitive to higher frequencies.
答案A
解析 1985年内华达德尔卢兹火山爆发引起火山泥流的例子是用来说明[A]声学流动监控器在预测火山泥流方面非常有效。[B]世界各地都在使用声学流动监控系统。[C]声学流动监控器可以完全避免火山泥流。[D]声学流动监控器对较高的频率很敏感。文章第二段以1985年发生在哥伦比亚的火山泥流为例，讲述了人们运用声学流动监控系统对火山泥流的预测。这样，监控系统就能听到很远之外的火山泥流，并能创造较多的宝贵时间进行撤离。该例是为了说明人们利用声学流动监控器对火山泥流的有效预测，因此，正确答案是[A]。该例无法说明[B]．[C]这个命题本身与原文不符；[D]是讲声学流动监控器的原理，不是对发生在1985年的火山泥流的说明。
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