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2012年12月英语六级阅读全真模拟题 The Hydrogen Economy While fossil fuels have played an important role in getting society to the point it is at today, there are four big problems that fossil fuels create. Air pollution-When cars burn gasoline, they would ideally burn it perfectly and create nothing but carbon dioxide and water in their exhaust. Unfortunately, the internal combustion(燃烧)engine is not perfect. In the process of burning the gasoline, it also produces carbon monoxide, a poisonous gas; nitrogen oxides, the main source of urban smog; and unburned hydrocarbons, the main source of urban ozone . Catalytic converters (催化式排气净化器) eliminate much of this pollution, but they arent perfect. Air pollution from cars and power plants is a real problem in big cities. It is bad enough now that, in the summer, many cities have dangerous levels of ozone in the air. Environmental pollution-The process of transporting and storing oil has a big impact on the environment whenever something goes wrong. An oil spill, pipeline explosion or well fire can create a huge mess. Global warming-When you burn a gallon of gas in your car, you emit about 5 pounds (2.3 kg) of carbon into the atmosphere. If it were solid carbon, it would be extremely noticeable-it would be like throwing a 5-pound bag of sugar out the window of your car for every gallon of gas burned. But because the 5 pounds of carbon comes out as an invisible gas, carbon dioxide, most of us are oblivious (不注意的)to it. The carbon dioxide coming out of every cars tailpipe is a greenhouse gas that is slowly raising the temperature of the planet. The ultimate effects are unknown, but it is a strong possibility that, eventually, there will be dramatic climate changes that affect everyone on the planet. For example, if the ice caps melt, sea level will rise significantly, flooding and destroying all coastal cities in existence today. Thats a big side effect. | 
