阅读下面短文，按照句子结构的语法性和上下文连贯的要求， 在空格处填入一个适当的词或使用括号中词语的正确形式填空。
The world____ (it) is becoming smaller and smaller with modern traffic, TV, telephone and so on. Life today is much easier ____it was hundreds years ago, but it has also brought new problems. One of the biggest ____is pollution. Pollution comes in many ways. We can see it, smell it, drink it and even hear it.
The more people, the more pollution. Many years ago, the problem was not so serious because there were not so many people. Man are____(pollute) the whole word now.
Air pollution is still the most serious. It's bad to all living things in the world. Water pollution kills our fish and pollutes our drinking water. Noise pollution makes us angry more____(easy).
Many countries are paying more and more attention ____ pollution. They make rules to stop people from____(burn) coal(煤炭) in houses and factories in the city.
The pollution of CO2 is now the most dangerous problem of air pollution. It ____(cause) by heavy traffic. It is sure that if there are ____people driving, there ____(be) less air pollution.
2NH3(g) 。