[填空题]Trees are useful to man in three important ways: they provide him with wood and other products; they give him shade; and they help to prevent (26) and floods. Unfortunately, in many parts of the world, man has not realized that the third of these services is the most important.
Two thousand years ago, a rich and (27) country cut down its trees to build (28) , with which to gain itself an empire. It gained the empire, but without its trees, its soil became hard and poor. When the empire fell to pieces, the home country found itself faced by floods and (29) . Even when a government realizes the (30) of trees, it is difficult for it to (31) the villagers to see this. For the villagers want wood for cooking, heating and even earning money by selling wood to town’s men. They are usually too (32) to plant and look after new trees. So, unless the government has a good system of control, or can educate the people, the forest

[单项选择]Trees are useful to man in three important ways: they provide wood and other products; they give him shade; and they help to prevent droughts and floods.
Unfortunately, in many parts of the world, man has not realized that the third of these services is the most important. In his eagerness to make money from trees, he has cut them down in large numbers, only to find that without them he has lost the best friends he had. And besides, he is usually too careless to plant and look after new trees. So the forests slowly disappear.
This does not only mean that man will have fewer trees. The results are even more serious: for where there are trees, their roots break up soi — allowing the rain to sink in — and also bind the soil, thus preventing it from being washed away easily; but where there are no trees, the rain falls on hard ground and flows away, causing floods and carrying away the rich top-soil. When all the top-soil is gone, nothing remains but worthless desert.
What is