完形填空

先阅读短文，掌握其大意，然后从A、B、C、D四个选项中选出一个可以填入相应空白处的最佳答案．

　　There are seas, lakes and forests on the earth. There are cities, towns and farms, 1 . All these things are environments(环境). The environment is everything around you. It can be living(有生命的), like a forest. It can be non-living(无生命的), 2 a rock or a mountain.

　　No living thing can live 3 . Every living thing depends(依靠) on 4 things in its environment. When something 5 , it affects(影响) something else. If a non-living thing like water becomes 6 to find, plants will be affected. If all plants 7 , the animals that eat plants will be affected.

　　Weather, such as(例如) temperature, wind and rain, can change an environment. 8 can change an environment, too. Some things that people do cause(引起) 9 change or only a small change in the environment. For example, people can 10 the forest and just travel around. They can cut down only a few trees and 11 change the environment very much. On the other hand(另一方面), some things that people do cause 12 change. Sometimes they can change 13 environment by cutting down a forest.

　　 14 changes are bad. If we cut down a forest to build houses or plant crops(庄稼), this may not be 15 . But what would happen if we cut down all trees in all forests 16 these things or if we didn't 17 some and plant something else?

　　It is clear that people are able to 18 an environment. This is why it is important to think about the changes before we make 19 . Let's all work together to 20 it clean and healthy.

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