短文填空
请用适当的词完成下面的短文，每个空只能填写一个形式正确、意义相符的单词。
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.