In some countries in which there is very little rain at any time, the farmers have to irrigate their fields. Irrigation is easy enough if there is a great river near the crops, and if there is plenty of water in it. Canals carry the river water to the fields when necessary. In some seasons there may be too much water in the river, and the water may flood all the land near it. At other times the river water may not be enough for all the farmers, and then it will be difficult to irrigate the fields.
To prevent these difficulties, a dam may be built across the river. This will store water for dry seasons, and in wet seasons it will prevent the flooding of the land. A great lake will, no doubt, be formed behind the dam, and this may mean that houses and fields will be covered with water. Most people will accept that and will be ready to move to other places. Then the country can have the right amount of water at all times; more crops will be grown, there will be more food for everyone, and there will be no more floods.
Dams have been built for centuries in different parts of the world. Modern dame are usually built of concrete（混凝土）but earth dams were used in India and Sri Lanka about 2500 years ago. There was another across the River Tigris very long ago, and large numbers of irrigation canals were made in Iraq to irrigate the land. The Romans were great dam-builders and there was also a famous dam at Ma’ rubin Arabia.
We cannot use sea water for irrigation; the salt will not allow the crops to grow. Fresh water is always needed, and it is very difficult and costly to make fresh water from sea water. This is done in a few countries, but the fresh water is for drinking. There is not enough at present for irrigation. Perhaps a scientist will one day invent a cheaper way of producing fresh water from the sea.
【小题1】The first paragraph mainly talks about .A．the problems caused by river waterB．the necessity to irrigate the fieldC．the water shortage in some countriesD．the canals carrying water to fields【小题2】A dam is usually built .A．to cover the houses and fieldsB．for people to have the right amount of water at all timesC．to grow crops betterD．to form a large lake【小题3】The underlined word“that”means .A．a dam will be built across the riverB．a dam will store water for dry seasonsC．in wet seasons a dam will prevent the flooding of the landD．houses and fields will be covered will water【小题4】The third paragraph suggests that .A．modern dams are made of concreteB．Romans once built many great damsC．there was too much water in 1rap in the pastD．modern dams are better than early damsABDB
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(答案→)ABDB
解析：本文讲述了大坝蓄水灌溉的知识，并仔细分析了建筑大坝的好处等。
【小题1】A 段落大意题。在第一段中主要讨论的是河水灌溉的问题，有了大量的河水，人们就可以进行灌溉，要是河水不够了，那么灌溉就成了问题，故A项正确。
【小题2】B 细节题。根据第二段前3行To prevent these difficulties, a dam may be built across the river. This will store water for dry seasons, and in wet seasons it will prevent the flooding of the land.说明大坝可以蓄水供人们在任何时候来使用，故B项内容正确。
【小题3】D 推理题。根据第二段2,3,4行A great lake will, no doubt, be formed behind the dam, and this may mean that houses and fields will be covered with water. Most people will accept that and will be ready to move to other places.说明这里的that就是指前一句的大坝后面的房子和土地会被大水淹没，故D项正确。
【小题4】B 推理题。根据本段最后一句The Romans were great dam-builders and there was also a famous dam at Ma’ rubin Arabia.说明罗马人是建筑大坝的能手，故说明他们曾经修筑过多个大坝所以才能这样说，故B项正确。