People and the sources of air pollution are found in the same places. This means that cities with large populations have the biggest problem of dirty air. Air pollution is caused by many different things. A major source of air pollution is the gas fumes from cars. Statistics show that 93 percent of all auto trips are within cities. Another major source of dirty air is the burning of coal and oil for energy. This energy is needed to make electricity. Of course, much more electricity is used in the city than in the country.
On the average, we throw away more trash and garbage than the year before. The burning of garbage contributes to air pollution. Many major industries are also responsible for the dirty air in the around cities. The fumes from iron, steel, chemical, and petroleum production add particles to the air.
The effects of air pollution range from mild headaches to death. The levels of pollution found in heavy for traffic may cause headaches for loss of clear vision. Wherever coal and oil are used for fuel, fumes may kill trees and plants and cause metal to corrode. In some of the larger cities, these fumes endanger the live of human beings by contributing to lung diseases and causing early death.
15．The key point of the passage is that _________.
A. the cause of air pollution is people
B. the causes and the effects of air pollution are both found in cities
C. the effects of air pollution range from headaches to death
D. air pollution is caused by dirty air
16．What is the purpose of this passage?
A. to persuade people to stop polluting the air
B. to tell the causes of air pollution
C. to tell why cities are bad places to live
D. to describe why cities are bad places to live
17. Why is air pollution more grave in the city than in the country?
A. Because there larger populations in cities.
B. Because the air in the city is dirty.
C. Because there are more cars in the city.
D. Because there are much more gas fumes from cars and burning of coal and oil for energy  
in the city than in the country.
18．The fumes from coal and oil may cause metal ______.
A. to turn black                              B. to become rusty            
C. to corrode                            D. to twist
19. In larger cities, the fumes from coal and oil may contribute to ___________.
A. heart disease                      B. lung diseases   
C. loss of clear vision              D. serious headache