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  3. Since ancient times volcanoes have struck terror and wonder into the heart of man. Yet there is much more to volcanic activity t

Since ancient times volcanoes have struck terror and wonder into the heart of man. Yet there is much more to volcanic activity t

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问题     Since ancient times volcanoes have struck terror and wonder into the heart of man. Yet there is much more to volcanic activity than the spectacular and awe-inspiring eruptions which sometimes cause great destruction. Comparatively few volcanoes are really dangerous and many of them are merely "noisy neighbors" to those who live nearby, causing only a slight nuisance. The share of good which volcanic force has brought to the world has too often been obscured by the violent outbursts of a few highly active ones.
    In recent geological times, volcanoes have thrown up a multitude of islands dotted among the oceans and seas of the world which have added some hundreds of square miles to the land surface of the earth. Their real value to man, however, lies in the widely varying use to which they can be put. On the continental land masses volcanoes have raised mountains which have done much to increase what would otherwise have been a scanty rainfall. And the peoples of the tropics often have a lofty volcanic cone to thank for a more temperate climate.
    The greatest benefit so far brought by volcanoes, however, has been the fertilizing effect of the lava and ash they eject. The soils formed by the decay of these volcanic products yield rich harvests in many countries. This fertility accounts for the amazing density of population in the neighborhood of many active craters. Forgetful of past catastrophes and heedless of the dangers that still threatens them, farmers have settled in large numbers round Etna and Vesuvius and volcanoes in Central America, Indonesia, the Philippines, and Japan, but they now run fewer risks than their forefathers. In the past few decades observation posts have been set up alongside the most dangerous craters, so that all impending eruption can be detected and warning given in good time.
    For a long while no attempt was made to harness the enormous power generated by volcanoes. Even in quite subsidiary forms it was put only to such domestic use as heating and cooking, or was employed therapeutically in the form of hot mineral springs. Today the remarkable technical achievements of Iceland and Italy are serving as a spur to the harnessing of geothermal power for use in industry. In a few years’ time, other countries, particularly those with few other sources of natural power, are likely to be drawing more and more on the inexhaustible power 35 offered by volcanoes.
Observation posts have been set up alongside the most dangerous craters to ______.
选项
答案to detect and warn people of imminent eruptions
解析 “observation posts”在第三段最后一句中有所提及，句中的“so that”引导目的状语从句，正好是答案。但文中的被动语态句如何转换成主动词组来答题，可能有些难度。
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