Trees are useful to man 【B1】______ three very important ways: they provide him 【B2】______

wood and other produces; they help to check soil erosion and they help to 【B3】______ drought and floods.

【B4】______ , in many parts of the world, man has not realized that the 【B5】______ of these services is the most important. In his eagerness to draw quick profit from the trees, he has cut them down in large 【B6】______ , only to find that 【B7】______ them he has lost the best friend he had.

Two 【B8】______ years ago a rich and powerful country cut down its trees to build warships 【B9】______ to gain itself an empire. It gained the empire, but without its trees, its soil became hard and poor. When the empire 【B10】______ to pieces, the home country found itself faced by floods and starvation.

【B11】______ a government realizes the importance of a plentiful supply of trees, it is difficult for it to persuade the villager to see this ,the villager wants wood to cook his food 【B12】______ ; and he can earn money by making charcoal or selling wood to the townsman. He is usually too lazy or too 【B13】______ to plant and look after new trees. So, 【B14】______ the government has a good system of control, or can educate the people, the forests slowly disappear.

This does not only mean that the villagers' sons and grandsons have 【B15】______ 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, thus preventing 【B16】______ washed away on the surface, causing floods and carrying 【B17】______ ground away 【B18】______ it the rich top-soil, in which crops grow so well. When all the top-soil is 【B19】______ , nothing remains 【B20】______ worthless desert.

A．with

B．at

C．in

D．through