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For millions’ of years we have known a world whose resource seemed illimitable however fast, we cut down trees, nature unaided would replace them. However many fish we took from the sea, nature would restock it. However much sewage we dumped into the river, nature would purify it, just as she would purify the air, however much smoke and fumes we put into it. Today we have reached the stage of realizing that rivers can be polluted past praying for, that seas can be overfished and the forests must be managed and fostered if they are not to vanish.
But we still retain our primitive optimism about air and water. There will always be enough rain falling from the skies to meet our needs. The air can absorb all the filth we care to put in it. Still less do we worry whether we could ever run short of oxygen. Surely there is air enough to breathe. Who ever asks where oxygen comes from, to begin with They should--for we now consume about 10 percent of all the atmospheric oxygen every year, thanks to the many forms of combustion which destroy it; every car, aircraft and power station destroys oxygen in quantities far greater than men consume by breathing.
The fact is we are just beginning to press up against the limits of the earth’ s capacity. We begin to have to watch what we are doing to things like water and oxygen, just as we have to watch whether we are overfishing or overfelling. The realization has dawned that the earth is t spaceship with strictly limited resources. These resources must, in the long run, be recycled, either by nature or by man. Just as the astronaut’ s urine is purified to provide drinking water and just’ as his expired air is regenerated to be breathed anew, so all the earth’ s re sources must be recycled, sooner or later. Up to now, the slow pace of nature’ s own recycling has served, coupled with the fact that the "working capital" .of already recycled material was large. But the margins are getting smaller and if men, in even larger numbers, are going to require even larger quantities, the pace of recycling will have to be artificially quickened.
All we have is a narrow band of usable atmosphere, no more than seven miles high, a thin crust of land, only one eighth of the surface of which is really suitable for people to live on, and a limited supply of drinkable water, which we continually reuse. And in the earth, we have a capital of fossil fuels and ores, which, we steadily run down billions of times faster than nature, restores it. These resources are tied together in a complex set of transactions. The air helps purify the water, the water irrigates the plants, the plants help to renew the air.
We heedlessly intervene in these transactions. For instance, we cut down the forests, which transpire water and oxygen, we build dams and pipeline which limit the movement of animals, we pave the earth and build reservoirs, altering the water cycle. ’So far, nature has brushed off these injuries as pinprick. But now we are becoming so strong, so clever and so numerous, that they are beginning to hurt.

The analogy drawn between the earth and a spaceship is to illustrate the idea of（）on earth.

A:unlimited resources B:limited resources C:recycling D:regeneration

A very important world problem is the increasing number of people who actually inhabit this planet. The limited amount of land and land resources will soon be unable to support the huge population if it continues to grow at its present rate.
So why is this huge increase in population taking place It is really due to the spread of the knowledge and practice of what is becoming known as "Death Control". You have no doubt heard of the term "Birth Control". "Death Control" is something rather different. It recognizes the work of the doctors and scientists who now keep alive people who, not very long ago, would have died of a variety of then incurable diseases. Through a wide variety of technological innovations that include farming methods and the control of deadly diseases, we have found ways to reduce the rate at which we die. However, this success is the very cause of the greatest threat to mankind.
If we examine the amount of land available for this ever-increasing population, we begin to see the problem. If everyone on the planet had an equal share of land, we would each have about 50,000 square meters. This figure seems to be quite encouraging until we examine the amount of usable land we actually have. More than three fifths of the world’s land cannot produce food.
Obviously, with so little land to support us, we should be taking great care not to reduce it further. But we are not! Instead, we are consuming its "capital’—its nonrenewable fossil fuels and other mineral deposits that took millions of years to form but which are now being destroyed in decades. We are also doing the same with other vital resources not usually thought of as being nonrenewable such as fertile soils, groundwater and the millions of other species that share the earth with us.
It is a very common belief that the problems of the population explosion are caused mainly by poor people living in poor countries who do not know enough to limit their reproduction. This is not true. The actual number of people in an area is not as important as the effect they have on nature. Developing countries do have an effect on their environment, but it is the populations of richer countries that have a far greater impact on the earth as a whole.
In Paragraph 4 the writer implies that fertile soils are ______.

A:limited B:renewable C:nonrenewable D:productive

A very important world problem is the increasing number of people who actually inhabit this planet. The limited amount of land and land resources will soon be unable to support the huge population if it continues to grow at its present rate.
So why is this huge increase in population taking place It is really due to the spread of the knowledge and practice of what is becoming known as "Death Control". You have no doubt heard of the term "Birth Control". "Death Control" is something rather different. It recognizes the work of the doctors and scientists who now keep alive people who, not very long ago, would have died of a variety of then incurable diseases. Through a wide variety of technological innovations that include farming methods and the control of deadly diseases, we have found ways to reduce the rate at which we die. However, this success is the very cause of the greatest threat to mankind.
If we examine the amount of land available for this ever-increasing population, we begin to see the problem. If everyone on the planet had an equal share of land, we would each have about 50,000 square meters. This figure seems to be quite encouraging until we examine the amount of usable land we actually have. More than three-fifths of the world’s land cannot produce food.
Obviously, with so little land to support us, we should be taking great care not to reduce it further. But we are not! Instead, we are consuming its "capital" its nonrenewable fossil fuels and other mineral deposits that took millions of years to form but which are now being destroyed in decades. We are also doing the same with other vital resources not usually thought of as being nonrenewable such as fertile soils, groundwater and the millions of other species that share the earth with us.
It is a very common belief that the problems of the population explosion are caused mainly by poor people living in poor countries who do not know enough to limit their reproduction. This is not true. The actual number of people in an area is not as important as the effect they have on nature. Developing countries do have an effect on their environment, but it is the populations of richer countries that have a far greater impact on the earth as a whole.
In Paragraph 4 the writer implies that fertile soils are______.

A:limited B:renewable C:nonrenewable D:productive

Although a given waveform may contain frequencies over a very broad range, as a practical matter any transmission system will be able to accommodate only a limited band of (86) . This, in turn, limits the data rate that can be carried on the transmission (87) A square wave has an infinite number of frequency components and hence an infinite (88) However, the peak amplitude of the kth frequency component, kf, is only l/k, so most of the (89) in this waveform is in the first few frequency components. In general, any digital waveform will have (90) bandwidth. If we attempt to transmit this waveform as a signal over any medium, the transmission system will limit the bandwidth that can be transmitted.

（90）是（）

A:small B:limited C:infinite D:finite

Although a given waveform may contain frequencies over a very broad range, as a practical matter any transmission system will be able to accommodate only a limited band of （1）. This, in turn, limits the data rate that can be carried on the transmission （2）. A square wave has an infinite number of frequency components and hence an infinite （3）. However, the peak amplitude of the kth frequency component, kf, is only l/k, so most of the （4） in this waveform is in the first few frequency components. In general, any digital waveform will have（5） bandwidth. If we attempt to transmit this waveform as a signal over any medium, the transmission system will limit the bandwidth that can be transmitted.

（5）是（）

A:small B:limited C:infinite D:finite

Although a given waveform may contain frequencies over a very broad range, as a practical matter any transmission system will be able to accommodate only a limited band of （） . This, in turn, limits the data rate that can be carried on the transmission （） . A square wave has an infinite number of frequency components and hence an infinite （） .However, the peak amplitude of the kth frequency component, kf, is only 1/k, so most of the （） in this waveform is in the first few frequency components. In general, any digital waveform will have （） bandwidth. If we attempt to transmit this waveform as a signal over any medium, the transmission system will limit the bandwidth that can be transmitted.

any digital waveform will have （） bandwidth.

A:small B:limited C:infinite D:finite

A I am a writer．I spend a great deal of my time thinking about the power of language－the way itcan evoke(唤起)an emotion，a visual image，a complex idea，or a simple truth．Language is thetool of my trade．And I USe them all－all the English I grew up with. Bom into a Chinese family that had recently arrived in California，I’ve been giving more thoughtto thekind of English my mother speaks．Like others，I have described it to people as“broken”English．But feel embarrassed to say that．It has always bothered me that I can think of no way to describe it other than“broken”。as if it were damaged and needed to be fixed，as if it lacked a cer’tain wholeness．I，ve heard other terms used，“limited English，”for example．But they 8eem just as bad．as if everything is limited，including people’s perception(认识)of the limited English speaker. I know this for a fact．because when I was growing up，my mother’s“limited’’English limited my perception of her．1 was ashamed of her English．I believed that her English reflected the quality of what she had to say．That is，because she expressed them imperfectly her thoughts were imperfect．And I had plenty of evidence to support me：the fact that people in department stores，at banks．and at restaurants did not take her seriously，did not give her good service，pretended not to understand her，or even acted as if they did not hear her． I started writing fiction in l985．And for reason l won’t get into today，I began to write stories using aU the English I grew up with：the English she used with me，which for lack of a better term might be described as“broken”，and what I imagine to be her translation of her Chinese，her inter-nal(内在的)language，and for that I sought to preserve the essence，but neither an English nor a Chinese stnlcture：1 wanted to catch what language ability tests can never show；her intention，her feelings，the rhythms of her speech and the nature of her thoughts． What is the passage mainly about( )

A:The changes of the author’s attitude to her mother’s English． B:The limitation of the author’s perception of her mother． C:The author’s misunderstanding of“limited”English． D:The author’s experiences of using broken English．

A I am a writer．I spend a great deal of my time thinking about the power of language－the way itcan evoke(唤起)an emotion，a visual image，a complex idea，or a simple truth．Language is thetool of my trade．And I USe them all－all the English I grew up with. Bom into a Chinese family that had recently arrived in California，I’ve been giving more thoughtto thekind of English my mother speaks．Like others，I have described it to people as“broken”English．But feel embarrassed to say that．It has always bothered me that I can think of no way to describe it other than“broken”。as if it were damaged and needed to be fixed，as if it lacked a cer’tain wholeness．I，ve heard other terms used，“limited English，”for example．But they 8eem just as bad．as if everything is limited，including people’s perception(认识)of the limited English speaker. I know this for a fact．because when I was growing up，my mother’s“limited’’English limited my perception of her．1 was ashamed of her English．I believed that her English reflected the quality of what she had to say．That is，because she expressed them imperfectly her thoughts were imperfect．And I had plenty of evidence to support me：the fact that people in department stores，at banks．and at restaurants did not take her seriously，did not give her good service，pretended not to understand her，or even acted as if they did not hear her． I started writing fiction in l985．And for reason l won’t get into today，I began to write stories using aU the English I grew up with：the English she used with me，which for lack of a better term might be described as“broken”，and what I imagine to be her translation of her Chinese，her inter-nal(内在的)language，and for that I sought to preserve the essence，but neither an English nor a Chinese stnlcture：1 wanted to catch what language ability tests can never show；her intention，her feelings，the rhythms of her speech and the nature of her thoughts． What is the passage mainly about( )

A:The changes of the author’s attitude to her mother’s English． B:The limitation of the author’s perception of her mother． C:The author’s misunderstanding of“limited”English． D:The author’s experiences of using broken English．