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For those on a tighter budget or just eager to get a little healthful exercise, there is always the bicycle, which appears to be mounting a comeback. After a slump in the mid-1990s, bicycle production leaped to more than 100 million units in 2000, compared with just 62 million in 1980. But what about couch potatoes who have no willpower to pedal? At least two manufacturers, Aprilia in Italy and Manhattan Scientifics in the U.S., think they have the answer: make the venerable two-wheeler propel itself with its very own fuel cell. --With reporting by Joseph R. Szczesny/Detroit