为你解答.
People （have lived） on the earth for only 2,000,000 years . But the human beings （are changing） the planet in many ways . For example, we have done many bad things （to pollute） our planet . People （invented） cars so that we can go anywhere （easily） in a short time . Soon , there are more and more people （drive） cars. And the traffic becomes much （busier）. They have polluted the air. And many people in cities are in bad （health）. Can we stop people from driving cars ? Of course not. We should do something （useful） to make the pollution less and less. Try your （best） to help our planet.