Trees are useful to man in three important ways: they give him wood and other useful things, they give him cool places, and they help to stop drought and flood. Unluckily, in many parts of the world, man has not found that the third of these points is the most important. Man wants to make money from trees, so he has cut them down in large numbers, only to find that without them he has lost the best friends he had. And also, he is usually too careless to plant and look after new trees. So the forests slowly disappear. This does not only mean that man will have fewer trees. The results are even worse: for where there are trees, their roots break up soil-make the rain in-and also bind the soil, thus stopping it from being washed away easily; but where there are no trees, the rain falls on hard ground and flows away, causing floods and carrying away the rich top-soil. When all the top-soil is gone, nothing is left but useless desert. Land becomes desert after all trees are cut down because   _   .
roots of trees break up the soil there are too many rainfalls strong winds bring a lot of sand there are no longer trees to keep the rain and protect the top-soil.