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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(真菌) 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 (同温层) 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 miles. They are pretty sparse at such levels, about one for every two thousand cubic 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 文章第一段作者讲到18英里的高空发现了真菌和细菌，但相较于飞机通常高度的2～6英里更稀少，在飞机高度上每立方尺有50～100个细菌和真菌，由此推断出正确答案为B。
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