In some countries in which there is very little rain at any time, the farmers have to water 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(运河) 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 (大坝) 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 growing, 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 dams are usually built of concrete(混凝土) ,but earth dams were used in India and Sri Lanka about 2,500 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.
The first paragraph mainly talks about ______.

A. the problems caused by river water
B. the necessity to irrigate the field
C. the water shortage in some countries
D. the canals carrying water to fields