Câu hỏi:

Trees are useful to man in three very important ways: they provide him with 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 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 pieces, the country found itself faced by floods and starvation.

Even though 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 will 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.

(Source: www.cmr.com.cn/teach/tongkao/tksV200505_dxyyb_st.doc)

Trees are useful to man mainly in three ways, the most important of which is that they can _______.