单项选择题

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; the burrowing invertebrate (无脊椎动物) such as earthworms and nematodes (线虫类) 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, often luminous, creatures. Spores (孢子) of 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 NASA has detected them, in decreasing numbers, at heights up to 18 miles. They are pretty sparse at such levels, about one for every 2,000 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 a dormant (睡眠状态的) state. Marine microbes have been detected at the bottom of the deep Pacific trench, sometimes as deep as seven miles; they are certainly not dormant. 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 25 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 100 feet or so.
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 obviously often disturbed, yet 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 minutia (~) of the balance of nature.

The writer states that fungi and bacteria（）