根据短文内容，完成下列各题。

    Trees are useful to man in three very important ways. The first important way is that(A) they provide man with food and wood.Trees provide not only man with food, but also many animals with food. (B)If there are notrees, man and animals couldn't live on the earth. The second important way is that trees bring us shade. On a hot summer day, people hope to have a rest under the shade of a tree after they walk a long way. The third important way is that trees help to prevent droughts (干旱).

    However, in many parts of the world, man has not realized the third (C). _________People have cut trees down in large numbers. (D)In the end, they find that they have lost the best friends.