3. The world itself is becoming much smaller by using modem traffic and modern communication means (通讯设备). Life today is much easier than it was hundreds of years ago, but it has brought new problems. One of the biggest is pollution (污染). To pollute means to make things dirty. Pollution comes in many ways. We see it, smell it, drink it and even hear it. Man has been polluting the earth. The more people, the more pollution. Many years ago, the problem was not so serious because there were not so many people. When the land was used up（用完） 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 (法规) to fight pollution. They stop people from burning coal (煤) 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 pollution. It is caused (引起) by heavy traffic. It is 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 population at the same time. Pollution comes in many ways. We can even hear it. Here "it" means ____.