Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions.

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 or selling wood to the townsman. 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 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, 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.

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