Hundreds of years ago, life was much harder than it is today. People didn't have modern machines.

    There was no modern medicine 1.

    Life today2new problems. One of the biggest is pollution. Water pollution has made our rivers and lakes dirty. It kills our fish and affects (影响) our drinking water. Noise pollution makes us 3 louder and get angry much easier. Air pollution is the most serious kind of pollution. It affects 4 living thing in the world.

    Cars, planes and factories all pollute our air every day. Sometimes the pollution is5thick that it is like a quilt (被状物)over a city. This kind of quilt is called smog(烟雾).