These are useful to man in three important ways: They provide him with wood and other prod

ucts; They give him shade and they help to prevent drought (干旱) and floods. Unfortunately, in many parts of the world, in his eagerness to draw quick benefit from the tree, man has cut down a large number of trees. Even where a government realizes the importance of plentiful supply of trees, it is difficult for it to persuade the villagers to see this. So unless the government has a great system of control, or can educate the people, the forests slowly disappear.

The resaults are more serious: for where there are trees their roots break the soil allowing the rain to sink and also hold the soil, thus preventing it from being washed away easily, but where there are no trees, the soil becomes hard and poor.

The rain falls on hard ground and flows away on the surface, causing floods, and carry away with it the rich top soil in which crops grow so well. When all the top soil is gone, nothing remains but worthless desert.

The purpose that the writer wrote this article for is ______.

A．to tell people that trees are very useful to man

B．to warn people not to cut down trees any more

C．to warn that man mustn't destroy forests any more

D．to explain how trees help to prevent drought and floods