Now, imagine that instead of CO2, our cars putout horse ploppies that fell on the road. U.S. cars would be delivering the number of pounds of horse ploppies you just calculated, each year, to each square foot of paved road in the country. The CO2 from our cars, if turned to horse ploppies, would make a one-inch-thick layer spread across all the paved roads in the entire country each year. Just take a moment and think of this—what would happen if you stomped on the accelerator in an inch of recycled hay? How about braking? After a few decades, would all the roads look like pickup-truck commercials, with giant sprays of something like mud coming off the tires? Would you enjoy being a pedestrian? Would joggers switch to cross-country skiing? To get total U.S. CO2 production, you need to multiply again by about 3—we heat and air condition our homes, etc., as well as driving our cars, and most of the heating and cooling comes from fossil fuels, too. So, spread that inch of horse ploppies across your living room carpet, and across every other living space in the nation. Put differently, the average American generates 15 tons of CO2 per year. (Compare this to a bit over half a ton per person per year of solid waste we in the USA put out in garbage cans to go to landfills.) If you had an inch-thick layer of horse ploppies each year on every square inch of paved road in America, you very clearly would smell it everywhere—the volatile organic molecules wafting off the mess would quickly be blown around the country and the world. We don’t smell the CO2, but it is everywhere, building up steadily in the atmosphere, changing the climate… and we humans clearly are influential enough to do this. So, for you alert readers playing along at home, how thick would the layer be if all the CO2 released by U.S. cars were converted to an equivalent weight of horse ploppies and spread uniformly across all the paved roads in the U.S.? (You've already seen this answer several times, so this is your reward for staying with us to the end.)单项选择题