Environmental problems are becoming more and more serious all over the world. With the development of industry and agriculture, cars make great noises and give off poisonous gas. Trees on the hills have been cut down,and waste water is being poured continuously into rivers. Furthermore,wherever we go today, we can find rubbish carelessly disposed. The whole ecological balance of the earth is changing. Massive destruction of environment has brought about negative effects and even poses a great threat to man's exis
