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 drought 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, only to find that without them he has lost the best friends he had.
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 hard and poor. When the empire fell to pieces, the country found itself faced by floods and starvation.
Even though 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 with, and he can earn money by making charcoal(木炭) or selling wood to the townsman（城市居民）. He is usually too lazy or too careless to plant and look after 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 the villager’s sons and grandsons have 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 hold the soil, thus preventing it 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 carrying away with it the rich topsoil, in which crops grow so well. When all the topsoil is gone, nothing remains but a worthless desert.
The purpose that the writer wrote this article is to ______.         

A．tell people the use of wood

B．tell people the function of tree roots

C．warn that man mustn’t destroy forests any more

D．explain how trees help to prevent drought and floods

In the writer’s opinion, _____, or the forests will slowly disappear.

A．measures must be taken

B．people shouldn’t draw benefit from the tree

C．government must realize the serious results

D．unless trees never be cut down

According to the article we know it is _____ to prevent the forests from slowly disappearing.

A．necessary but impossible

B．necessary but difficult

C．impossible and unimportant

D．difficult and impossible

In the last two paragraphs the writer wanted to make it clear that _____ .

A．where there are no trees, the soil becomes poor but soft

B．where there are many trees, there are more floods

C．where there are no trees, the land might become desert slowly

D．floods will make the land become desert