Today some people call Amsterdam the“City of Bicycles”because it is a city which is flat and convenient(方便的)for bicycles. In the 1960s, a group of cycling fans had an i  46 . They believed that it would be better for everybody if o  47 bicycles were allowed in the city centre. They were hopeful that this would help to save energy, reduce pollution and provide free public transport. The group p  48 hundreds of bicycles white and placed them in public places around Amsterdam for people to use. A  49 was allowed to take them and use them for short journeys, whether he was a local or a foreigner. Wherever someone finished a journey, they would l  50 the bike there for someone else to use. The problem was that it didn’t work – t  51 took all the bicycles within weeks! H  52 , more than thirty years later，the“white bike”is back in town---this time with a computer chip(芯片)to r  53 its every move! There is a  54 less traffic in central Amsterdam. Thanks to the good ideas of lots of people, like the cycling fans in the 1960s, many people around the world have been enjoying city centre streets w  55 cars for many years.