It is only during the last few years that man has generally realized that in the world of nature a balance exists between all forms of life. No living thing can exist by itself: it is a part of a system (体系) in which all forms of life are joined together. If we change one part of the natural order, this will in its turn almost certainly bring about changes in some other part.
The cutting down of forests reduces the supply of oxygen. The killing of weeds and insects by chemicals leads to the wide spread poisoning of animals and birds. The throwing of waste products into the ocean harm to life in the sea, while waste gases change the chemical balance of the atmosphere.
And so we could go on, adding more examples, until in despair (绝望) we might feel like giving up the struggle to control and keep within limits these harmful human activities. Man is very clever at changing the world around him to satisfy his immediate needs, but not so clever at looking far ahead, or at thinking about what the future results of his action might be.
The first paragraph mainly tells us that __________.

A．everything in nature can’t exist without the help of human beings

B．no living thing can exist naturally

C．all living things in nature depend on each other

D．man has well known the importance of the balance for a long time

The examples given in the second paragraph are used to prove that __________.

A．it is very important to protect forests

B．there are some living things which can exist all by themselves without change

C．all forms of life belong to a system in which all the part can be changed for one another

D．we cannot change one form of life or matter without disturbing (扰乱) the balance of nature

In the second paragraph the underlined word “atmosphere” means __________.

A．the production of chemical factories	B．the gas in the outer space
C．the mixture of gases that surrounds the earth	D．the health of the human body

The third paragraph suggests that __________.

A．man shouldn’t think only about his immediate needs without enough thought on the future results

B．man is very clever in planning his distant future

C．man often feels that he has to give up in despair

D．man is always anxious to control and keep his actives within limits