People who live in heavily industrialized areas do not get as much sunlight as they should. Dust 【 C1 】 ______over 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 【 C2 】 ______about. 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 【 C3 】 ______around the earth, keeping warms in and so slowly raising the 【 C4 】 ______of the earth. This could really change the climate all over the world. Everybody wants to have a car. 【 C5 】 ______, 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 【 C6 】 ______. Did you know, for example, that a hundred cars produce on 【 C7 】 ______a third of a ton of CO, as well as many other 【 C8 】 ______substances 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 【 C9 】 ______new engines for cars. But what about the millions of cars already in use? Must they all be thrown away? It might be very 【 C10 】 ______to 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