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 part of a system in which all forms of life are joined together. If we change one part of the nature order, this will almost certainly bring about changes in some other parts. The cutting of forests reduced the supply of oxygen. The killing of weeds and insects by chemicals led to the wide-spread poisoning of animals and birds. The throwing of waste products into the ocean hurt life in the sea, while waste gases changed the chemical balance of the atmosphere and shut out some of the sun's necessary life-giving rays. And so we could go on adding more examples until in despair  we might feel like giving up the struggle to control these harmful human activities. Man is very clever at changing the world around him to satisfy his immediate needs, but he is not so clever at looking far ahead, or at thinking about what the future results of his action might be. Man may well destroy himself because of his silly action. Which of the following is true according to the passage?
Cutting down woods does little harm to human beings. Man has to pay much more attention to the future results of his present action. Oxygen comes from forests. The passage tells us to try our best to get as much as possible immediately.