Air pollution occurs when wastes dirty the air. People produce most of the wastes that cause air pollution. Such wastes can be in the form of gases or particles of solid or liquid matter. These substances result chiefly from burning fuel to power motor vehicles and to heat buildings. Industrial processes and the burning of garbage also contribute to air pollution. Besides, there are other polluting substances that exist naturally into the air. They include dust, soil particles, and naturally occuring gases. The rapid growth of population, industry and the increasing use of automobiles and airplanes have made air pollution a serious problem. The air we breathe has become so filled with pollutants that it can cause health problems. It also harms plants, animals, building materials, and fabrics. 13. All of the wastes that people produce cause air pollution. 14. Soil participles are natural pollutants. 15. Polluted air has a harmful influence on people, plants and animals. 16. Air pollution has become a serious problem due to the increasing population and industry. 17. The word “they” in line 5 refers to: A. motor vehicles B. natural pollutants C. Industrial processes D. buildings 18. What does not contribute to air pollution? A. The burning of garbage B. industrial processes C. motor vehicles D. the natural existence of oxygen in the air