The world itself is becoming much smaller by using modern traffic and modern communication. Life today is much easier than it was hundreds of years ago, but it has brought new problems. One of the biggest problems is pollution. To pollute means to make things dirty. Pollution comes in many ways. We see it, smell it, drink it and even eat it. Man has been polluting the earth. The more people, the more pollution. Many years ago, the problems was not so serious because there were not so many people. When the land was used up or the river was dirty in one place, man moved to another place. But this is no longer true. Man is now slowly polluting the whole world. Air pollution is still the most serious. It's bad for all living things in the world, but it is not the only one kind of pollution. Water pollution kills our fish and pollutes our drinking water. Noise pollution makes us angry more easily. Many countries are making rules to fight pollution. They stop people from burning coal in houses and factories in the city, and from putting dirty smoke into the air. Pollution by SO2 is now the most dangerous kind of air pollutions. It is caused by heavy traffic. We are sure that if there are fewer people driving there will be less air pollution. The earth is our home. We must take care of it. That means keeping the land, water and air clean. And we must take care of the rise in pollution at the same time. : Air pollution is the most serious kind of pollution because   _  .
it makes much noise it makes us angry more easily it makes our rivers and lakes dirty it's bad for all living things in the world.