阅读理解。 People and air pollution are often 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 kind of air pollution is the gas from cars. Statistics (统计资料) show that 93 percent of all auto trips are within cities. Another major kind 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 rubbish than the year before. The burning of rubbish makes air pollution even worse. Many major industries are also responsible for the dirty air in the around cities. The fumes (烟) from iron, steel, chemical, and petrol production add particles (微粒)  to the air. The effects of air pollution range from mild headaches to death. The levels of pollution found in heavy 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 (腐蚀). In some of the larger cities, these fumes endanger the live of human beings by contributing to lung diseases and causing early death. 1. Why is air pollution more in the city than in the country? A. Because there are 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. 2. The fumes from coal and oil may cause metal _____. A. to turn black B. to become rusty C. to corrode D. to twist 4. In larger cities,  the fumes from coal and oil may cause _____. A. heart diseases B. lung diseases C. loss of clear vision D. serious headaches 5. The main idea 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 6. What is the purpose of this passage? A. To tell people to stop polluting the air. B. To tell the causes of air pollution. C. To tell why cities are bad places to live in. D. To describe why cities are good places to live in.