The earth is 4 600 million years old. We humans have lived on earth for only 35 000 years, but during this period we have changed our planet a lot in many ways. Some things we've done are very good for the earth while some are bad. All over the world, people have cut down millions of trees. When it rains or when the wind blows, the earth is taken away. Forests have become deserts, so more and more sorts of animals and plants are disappearing. In big cities, cars and buses have polluted the air. Many people in cities now have health problems. Factories have also polluted the land and the water. As a result, many rivers and lakes are now dead. Around the earth, there's a special kind of oxygen called "ozone"(O3). It is important to the earth. But now air pollution is destroying it and that has made a very big hole in the ozone layer. Too much harmful radiation from the sun passes through the hole and reaches the earth directly. This is very dangerous because this kind of radiation can cause cancer. Carbon dioxide (CO2) in the air has increased a lot. It mainly comes from burning oil, coal and wood. This has formed a "blanket" around the earth. The heat from the sun can't escape so the temperature is rising. This is called "the greenhouse effect". It causes the level of the oceans to rise and the climate of the earth to change. All these problems are very serious, so we must do something now. ,A, B, C, D. The level of the oceans is rising because of   _  .
pollution few trees the greenhouse effect changes in the climate