Cheap coal ________ a lot of smoke．

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Biofuels

　　Biofules have been around as long as cars have．At the start of the 20th century, Henry Ford planned to fuel his Model Ts with ethanol(乙醇)．But discoveries of huge petroleum deposits(儲量)kept gasoline and diesel(柴油)cheap for decades, and biofucls were largely forgotten．However, with the recent rise in oil prices, along with growing concern about global warming caused by carbon dioxide emissions, biofuels have been regaining popularity．

　　Gasoline and diesel are actually ancient biofuels．But they are known as fossil fuels because they are made from plants and animals that have been buried in the ground for millions of years．Biofuels are similar, except that they're made form plants grown today．

　　Much of the gasoline in the United States in blended with a biofuel-ethanol．Countries around the world are using various kinds of biofuels．For decades, Brazil has turned sugarcane into ethanol, and some cars there can run on pure ethanol rather than as additive to fossil fuels．

　　On the face of it, biofuels look line a great solution．Since plants absorb carbon dioxide as they grown crops grown for biofuels should suck up about as much carbon dioxide as comes out of the tailpipes of cars that burn these fuels．And unlike underground oil reserves, biofuels are a renewable resource since we can always grow more crops to turn into fuel．

　　Unfortunately, it's not so simple．The process of growing the crops, making fertilizers and pesticides, and processing the plants into me!consumes a lot of energy．Also, because much of the energy used in production comes from coal and natural gas, biofiiels don't replace as much oil as they use．