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  3. Threes are useful to man in three very important ways：they provide him wood and other products；they give him shade；and they help

Threes are useful to man in three very important ways：they provide him wood and other products；they give him shade；and they help

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问题     Threes are useful to man in three very important ways：they provide him wood and other products；they give him shade；and they help to prevent drought (干旱) and floods.
    Unfortunately, in many parts of the world, man bas not realized that the third of these services is the most important. In his eagerness to draw quick profit from the trees, he has cut them down in large numbers.
    Two thousand years ago, a rich and powerful country cut down its trees to build warships, with which to gain itself an empire. It gained the empire but, without its trees, its soil became bare and poor. When the empire fell to pieced, the country found itself faced by floods and starvation.
    Even where a government realizes the importance of a plentiful supply of trees, it is difficult sometimes to make the people realize this. They cut down the trees but are too careless to plant and look after new trees. So, unless the government has a good system of control, or can educate the people, the forests slowly disappear.
    This does not only mean that there will be fewer trees. The results are even more serious：for where there are trees, their roots break the soil up, allowing the rain to sink in, and also bind the soil. This prevents the soil from being washed away. But where there are no trees, the rain falls on hard ground and flows away on the surface, and this causes floods and the rain carries away the rich topsoil in which crops grow. When all the topsoil is gone, nothing remains but worthless desert.
When there is a heavy rain, trees can help to prevent floods, as they can________.
选项 A、keep rain from falling down to soft ground
B、cause the soil to allow rainwater to sink in
C、prevent the soil from being washed away
D、make the topsoil stick together
答案B
解析 细节理解题。原文最后一段说明了树木的水土保持作用，树根切碎了泥土，让雨水渗入，从而保持了土壤，防止其流失，另一方面又防止雨水流走，形成洪水，所以选项B“树木通过让雨水流入封防止在大雨时形成洪水”是正确答案。
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