Trees are useful to man in three very important ways. The first important way is that they provide man ____ food, wood and other products(产品). Trees provide man and animals with food to live ____ the earth. It's not easy ____ man to live on the earth without trees.

    The ____ important way is that trees give us shade(树荫). On a hot summer day, people can have ____rest under the shade of a tree after they have walked a long way. You can imagine（想象） ____ important the shade of a tree is to man and animals.

    The third important ____ is that trees help to prevent droughts（干旱）and floods. However, in many parts of the world, man has ____ realized the third important way. He has cut ____trees in large numbers.____ the end, he finds that he has lost his best friends.