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  3. The biosphere is the name biologists give to the sort of skin on the surface of this planet that is inhabitable by living organi

The biosphere is the name biologists give to the sort of skin on the surface of this planet that is inhabitable by living organi

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问题     The biosphere is the name biologists give to the sort of skin on the surface of this planet that is inhabitable by living organisms. Most land creatures occupy only the interface between the atmosphere and the land; birds extend their range for a few hundred feet into the atmosphere; burrowing invertebrates such as earthworms may reach a few yards into the soil but rarely penetrate farther unless it has been recently disturbed by men. Fish cover a wider range, from just beneath the surface of the sea to those depths of greater than a mile inhabited by specialized creatures. Fungi and bacteria are plentiful in the atmosphere to a height of about half a mile, blown there by winds from the lower air. Balloon exploration of the stratosphere as long ago as 1936 indicated that moulds and bacteria could be found at heights of several miles, recently the USA’ s National Aeronautics and Space Administration has detected them, in decreasing numbers, at heights up to eighteen feet, compared with 50 to 100 per cubic foot at two to six miles(the usual altitude of jet aircraft), and they are almost certainly in an inactive state. Marine bacteria have been detected at the bottom of the deep Pacific trench, sometimes as deep as seven miles; they are certainly not inactive. Living microbes have also been obtained on land from cores of rock drilled(while prospecting for oil)at depths of as much as 1,200 feet. Thus we can say, disregarding the exploits of astronauts, that the biosphere has a maximum thickness of about twenty-five miles. Active living processes occur only within a compass of about seven miles, in the sea, on land and in the lower atmosphere, but the majority of living creatures live within a zone of a hundred feet or so. If this planet were scaled down to the size of an orange, the biosphere, at its extreme width, would occupy the thickness of the orange-colored skin, excluding the pith.
    In this tiny zone of our planet takes place the multitude of chemical and biological activities that we call life. The way in which living creatures interact with each other, depend on each other or compete with each other, has fascinated thinkers since the beginning of recorded history. Living things exist in a fine balance which is often taken for granted—for, from a practical point of view, things could not be otherwise. Yet it is a source of continual amazement to scientists because of its intricacy and delicacy. The balance of nature is obvious most often when it is disturbed, yet even here it can seem remarkable how quickly it readjusts itself to a new balance after a disturbance. The science of ecology—the study of the interaction of organisms with their environment—has grown up to deal with the minutiae of the balance of nature.
The writer states that fungi and bacteria______.
选项 A、are only found below the normal altitude of jet planes
B、have been found well at the normal altitude of jet planes
C、are not found below the surface of the earth
D、are mainly found below the surface of the earth
答案B
解析 第一段，作者谈到，美国研究发现fungi和bacteria在18英尺的高空含量明显减少，而且那里的细菌大多处于不活跃状态(inactive state)，而在2至6英尺高度，也就是与the usual altitude of jet aircraft相当的高度，每立方英尺则有50到100个细菌。四个选项中，只有B项“细菌在与喷气式飞机相当的高度能够被找到”符合文意。
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