缺词填空
Trees are u        to men in three important ways. They p     them with wood and other things. They give people shade in hot days ,and they help to prevent drought(干旱) and floods（洪水）.
S     , in many places of the world, men don’t know the third of these is the most important. They just want to make  money  from the trees. Some people cut down a large    n       of trees in the forests. Usually they are too busy c   down trees to plant and look after new  trees so the forests  slowly b      smaller and smaller.
This does not only m   that men will have fewer trees. The r     are even more serious. For where there are trees, their roots can prevent  the rain from w      away the soil, but if there are no trees, the rain falls on hard ground and wash away the rich top-soil (地表土). When the top-soil is gone, n       remains but death.