1970 was World Conservation Year. The United Nations wanted everyone to know that the world was in danger. They hoped something could be done. Here is one example of the problem. At one time there were 1,300 kinds of plants, trees and flowers in Holland ,but now only 866 are left. The others have been destroyed by modern people and their science. We are polluting earth, air, water and everything around us. We can't live without these things. If things go on like this, we shall destroy ourselves. What will happen in the future? Perhaps it's more important to ask "What must we do?" More and more young people have known this. Many of them are helping to save our earth. For example, they plant trees. In a small town in the United States, a large group of girls cleaned the banks of 11 kilometers of their river. Young people may hear about conversation through a song called "No one's going to change our world". It was made by Cliff Gichard and other singers. The money from it will help to conserve tigers, elephants and pandas on the earth. The United Nations hoped something could be done  _  .
to conserve the world to change the world to develop the world to destroy the world