D

　　In the atmosphere,carbon dioxide actd rather like a one-way mirror or the glass in the roof of a greenhouse which allows the sun's rays to enter but prevents the heat from excaping.

　　According to a weather expert's prediction,the atmosphere will be 3 warmer in the year 2050 than it is today, if man continues to burn luels at the present rate. If this warming up took place,the ice caps in the poles would begin to melt,thus raising sea level several meters and severely flooding coastal cities.Also，the increase in atmospheric temperature would lead to great changes in the climate of the northern hemisphere(半球),psooicbly resulting in an alteration of earth's chief food growing zones.

　　In the past,concern about a man - made warming of the earth has concentrated on the Arctic because the Antarctic is much colder and has a much thicker ice sheet.But the weather experts are now paying more attention to West Antarctic,which may be affected by a warming on the scale that will possibly take place in the next fifty years from on burning of fuels.

　　Satellite pictures showthat large areas of Antarctic ice are already disappearing.The evidence available suggests that a warming has taken place.This fits the theory that carbon dioxide warms the earth.

　　However,most of the fuel is burnt in the northern hemisphere,where temperatures seem to be falling.Scientists conclude,therefore,that up to now natural influences on the weather have exceeded those caused by man.The question is:Which natural cause has most effect on the weather?

　　One possibility is the variable behavior of the sun.Astronomers at one research station have studied the hot spots and cold spots(that is,the relatively less hot spots) on the sun. As the sun rotates(旋转),every 27.5 days,it presents hotter or colder faces to the earth,and different aspects to different parts of the earth.This seems to have a considerable effect on the distribution of the earth's atmospheric pressure,and consequently on wind circulation.The sun is also variable over a long term:its heat output goes up and down in cycles,the latest trend being downward.

　　Scientists are now finding mutual relations between models of solar weather interactions and the actual climate over many thousands of years ,including the last Ice Age.The problem is that the models are predicting that the world should be entering a new Ice Age and it is not.

　　One way of solving this theoretical difficulty is to assume a delay of thousands of years while the solar effects overcome the inertia(不活动，惰性) of the earth's climate.If this is right,the warming effect of carbon dioxide might thus be serving as a useful counter balance to the sun's diminishing heat.

　　68.It can be concluded that a concentration of carbon dioxide in the atmosphere would ___.

　　　A.mean a warming - up im the Arctic

　　　B.raise the temperature of the earth's surface

　　　C.prevent the sun's rays from reaching the earth's surface

　　　D.account for great changes in the climate in the northern hemiphere

　　69.Although the fuel consumption is greater in the northern hemisphere,temperatures there seem to be falling.This is _______.

　　　A.mainly because the levels of carbon dioxide are rising

　　　B.partly due to variations in the output of solar energy

　　　C.possibly because the ice caps in the poles are melting

　　　D.exclusively due to the effect of the inertia of the earth's climate

　　70.On the basis of their models,scientists are of the opinion that ________.

　　　A.the climate of the world should be becoming cooler

　　　B.the new Ice Age will be delayed by the greenhouse effect

　　　C.the man - made warming effect helps to increase the solar effects

　　　D.it will take thousands of years for the inertia of the earth's climate to take effect

It’s what he did _________ what he said that moved us.　 A. except for 　 B. but so

　 C. instead 　 D. rather than

It’s what he did _________ what he said that moved us.

　 A. except for 　 B. but so  C. instead 　 D. rather than

.The result of the test was rather _______.　

Sustainable development is applied to just about everything from energy to clean water and economic growth, and as a result it has become difficult to question either the basic assumptions behind it or the way the concept is put to use．This is especially true in agriculture, where sustainable development is often taken as the sole measure of progress without a proper appreciation of historical and cultural perspectives．
To start with, it is important to remember that the nature of agriculture has changed markedly throughout history, and will continue to do so ．Medieval agriculture in northern Europe fed, clothed and sheltered a predominantly rural society with a much lower population density than it is today．It had minimal effect on biodiversity, and any pollution it brought about was typically localized．In terms of energy use and the nutrients captured in the product it was relatively inefficient．
Contrast this with farming since the start of the industrial revolution．Competition from overseas led farmers to specialize and increase yields．Throughout this period food became cheaper, safe and more reliable．However, these changes have also led to habitat loss and to diminishing biodiversity．
What’s more, demand for animal products in developing countries is growing so fast that meeting it will require an extra 300 million tons of grain a year by 2050，yet the growth of cities and industry is reducing the amount of water available for agriculture in many regions．
All this means that agriculture in the 21st century will have to be very different from how it was in the 20th．This will require radical（激进的）thinking．For example, we need to move away from the idea that traditional practices are inevitably more sustainable than new ones．We also need to abandon the notion that agriculture can be “zero impact”．The key will be to abandon the rather simple and static measures of sustainability, which centre on the need to maintain production without increasing damage．Instead we need a more dynamic interpretation, one that looks at the pros and cons of all the various way land is used．There are many different ways to measure agricultural performance besides food yield: energy use, environmental costs, water purity, carbon footprint and biodiversity．It is clear, for example, that the carbon of transporting tomatoes from Spain to the UK is less than that of producing them in the UK with additional heating and lighting, but we do not know whether lower carbon footprints will always be better for biodiversity．
What is crucial is recognizing that sustainable agriculture is not just about sustainable food production．
72．How do people often measure progress in agriculture?
A．By its productivity                 B． By its impact on the environment　　
C．By its sustainability                D．By its contribution to economic growth
73．What does the author think of traditional farming practices?
　　A．They have remained the same over the centuries
　　B．They have not kept pace with population growth
　　C．They are not necessarily sustainable
　　D．They are environmentally friendly
74．What will agriculture be like in the 21st century?
　　A．It will go through radical changes
　　B．It will supply more animal products
　　C．It will abandon traditional farming practices
　　D．It will cause zero damage to the environment
75．What is the author’s purpose in writing this passage?
　　A．To remind people of the need of sustainable development
　　B．To suggest ways of ensuring sustainable food production
　　C．To advance new criteria for measuring farming progress
　　D．To urge people to rethink what sustainable agriculture is．