单项选择题

The dark smoke that comes out of stacks or from a burning dump contains tiny bits of solid or liquid matter. The smoke also contains many gases, most of which cannot be seen. Altogether, they make up the serious problems of air pollution. In so many places it keeps us from seeing the sun, irritates our eyes, causes us to cough, and makes us ill.
Air pollution can spread from city to city. It even spreads from one country to another. Some northern European countries have had "black snow" from pollutants that have traveled through the air from other countries and have fallen with the snow. So air pollution is really a global problem.
Air pollution can kill babies, older people , and those who have respiratory (呼吸道的) diseases. In London, in 1952, four thousand people died in one week as a result of a serious air-pollution episode. In 1948, in the small town of Donora, Pennsylvania, twenty people died in a four-day period of bad air pollution.
At levels often found in cities, air pollution increases the risks of certain lung diseases, such as emphysema, bronchitis, and asthma. Of course, smoking and other factors help to cause these illnesses, too, but these cases have increased greatly during recent years as air pollution has become worse. Air pollution can cause both airplane and auto accidents because it cuts down visibility. There are other possible health dangers from air pollution that we don’t know much about. For example, scientists are trying to find out whether chemicals that reach us from the air may cause changes in our cells.
These changes might cause babies to be born with serious birth defects. Scientists are trying to learn how all the many chemicals we are apt to take into our bodies from air, water, food, and even medicines act together to affect our health and the way our bodies work. That is another reason why it is so important to begin to control pollution now instead of waiting until we learn all the answers.
Air pollution costs us a lot of money. It soils and corrodes our buildings. It damages farm crops and forests. It has a destructive effect on our works of art. The cost of all this damage to our government is astronomical. It would be much more worthwhile, both for us and for the environment, to spend our tax dollars on air pollution control.
We can conclude that ______.