A、Power plants burn coal. B、Factories pour sewage. C、People bury trash underground. D、City population increases greatly. A

Clean air is important to good health. If the air contains impurities, they may be absorbed by our bodies and make us ill. We need clean air, but unfortunately, air pollution is generally present, especially in cities.
    Our cities have many factories, which we need to make food products, clothing and many other things. Every day these factories pour millions upon millions of tons of smoke and soot into the air. Power plants that burn coal add greatly to air pollution.
    Things made in factories wear out after a while and are thrown away as trash. We burn a lot of trash. More smoke and soot. And then there are the cars made in factories. Once they are out on the street, the cars will take in air and replace it with poisonous gases. Again, more smoke and soot.
    No area in the world is completely free of air pollution. We must take measures to control it. Now, more and more people are realizing the importance of clean air. Schools are now teaching about the pollution problem. Industries are beginning to help to clean up. They are installing equipment to clean up their smoke. Scientists and inventors are trying to develop cleaner engines for cars and trains. Someday we may drive cars with electricity. A new kind of paper that will dissolve in water and need not to be burned. A new kind of glass bottle that will melt in sunlight and disappear is being developed in some countries.
    Surely the day will come when people will be able to breathe clean air in cities.
Questions 19 to 22 are based on the passage you have just heard.
20. What has caused air pollution according to the speaker?
21. What kind of measure is being carried out to control air pollution?
22. What is the speaker’s attitude toward air conditions in the future?