A、To let out clean gas. B、To take in harmful gas. C、To use various plants. D、To let the air flow freely. C短文未句明确提到，使空气清新的最有效方法就是

For years scientists have been worried about the effects of air pollution on the earth’s natural conditions. Some believe the air inside many houses may be more dangerous than the air outside. It may be one hundred times worse. Indoor air pollution can cause a person to feel tired, to suffer eye-pain, headache and other problems. Some pollutants can cause breathing disorders, diseases of blood and even cancer. Most scientists agree that every modern house has some kind of indoor pollution. People began to notice the problem in the early 1970s. When builders began making houses and offices, they did not waste energy.[21]To do this they built buildings that limited the flow of air between inside and outside. They also began using man-made building materials. These materials are now known to let out harmful gases. As the problem became more serious, scientists began searching for a way to deal with it. They discovered a natural pollution control system for buildings—green plants. Scientists do not really know how plants control air pollution. They believe that a plant’s leaves absorb or take in the pollutants. In exchange the plant lets out oxygen through its leaves and through its tiny organizations on its roots. Scientists suggest that all buildings should have one large plant or several small plants inside for every nine square meters of space. Studies of different plants show that each absorbs different chemicals. So the most effective way to clean the air is to use different kinds of plants.
20. According to the speaker, what problem may indoor air pollution bring to people?
21. What causes the indoor air pollution according to the speaker?
22. What is the most effective way to clean the air?