题目

科学技术的发展是渐变和突变的统一。宋元时期科技取得突破性进展表现在                     （    ）                ①指南器具    ②印刷技术    ③造纸技术    ④火药武器                                     图1     A．①②③               B．②③④                C．①②④                D．①③④The War Against Sand Imagine living on the edge of a vast desert, which is moving quietly closer to your village every day and covering your fields. The desert is on the move. This is called desertification. Desertification occurs in regions close to an already existing desert.     71     The first is over-use of water in the area. There is not enough water in any case, and if it is not carefully used, disaster can follow. As time goes on, water shortages make farming more and more difficult. In some places, locals can remember local lakes and marshes which were once the homes for all kinds of fish and birds.     72     Farmers leave the land, and fields are replaced by deserts. The second cause is misuse or over-use of the land.     73     Ploughing(耕) large fields and removing bushes and trees means that the wind will blow away the soil. Once the soil is lost, it is hard to replace, and if there is rain, it has nowhere to go, and brings no benefit.     74     Every spring, the skies over some of eastern cities, thousands of kilometers away from the deserts, can be darkened by sandstorms. Dust from deserts can have a great effect on weather systems. While desertification is perhaps being partly caused by global warming, these sandstorms can make global warming worse by adding to what is known as the greenhouse effect. What can be done to slow down or stop the process of desertification? A great deal of work is already under way. Obviously first steps are to find new water sources.     75     Some types of grass also hold the soil together, and stop the wind taking it. Without these efforts, it will be harder and harder to stop the world’s deserts in their tracks, and more and more farmers will give up and head for cities. The lesson to be learnt lies beneath the sand. A. It generally arises from two related causes. B. It is not only the farmers and villagers who suffer. C. They have been completely buried by the sand now. D. Scientists are doing the research on the cause of desertification. E. They destroy the land, as the soil dries out and is then blown away. F. Tree planting can help, by providing barriers between desert and rich field. G. This means that the wrong crops are planted, and need more water than is available.