People who live in heavily industrialized areas do not get as much sunlight as they should. Dust 1over a city at altitudes (海拔) of between 4,000 and 8,000 feet cuts out between 20 and 50 percent of the sunlight and up to 90 percent of the ultraviolet light (紫外线). But dust is not the only thing to 2about. When fuels are burned, whether in a factory, or in a home, or in the engine of a car, CO ₂ is given off. CO ₂ in the atmosphere acts as a kind of 3around the earth, keeping warms in and so slowly raising the 4of the earth. This could really change the climate all over the world. Everybody wants to have a car. 5, while they are giving us so much pleasure, cars are also poisoning us. The amount of dangerous substances sent out by a car is quite 6. Did you know, for example, that a hundred cars produce on 7a third of a ton of CO, as well as many other 8substances every day? Now, more and more people are realizing the importance of clean air. Schools are now teaching about the pollution problems. Scientists and engineers are trying to 9new engines for cars. But what about the millions of cars already in use? Must they all be thrown away? It might be very 10to change them so that they, too, are pollutiorvfree. A. forge B. screen C. expensive D. floating E. unfortunately F. average G. poisonous H. develop I. temperature J. surprising K. overtaking L. essential M. regulation N. generously O. worry