下面的短文有15处空白，请根据短文内容为每处空白确定1个最佳选项。 London Cabbies（出租车司机） Every city in the world has taxis to take tourists to interesting places.London is the only city in the world where taking a taxi is an （51）experience for tourists.This is partly because of the special black cabs,which are found in no other country.But it is also because of the drivers themselves.（52）British people are famous for being polite and reserved,London cabbies are well-known（53）their willingness to talk. Some customers say that once the door shuts and the cab（54）off they are a captive（监禁了的）audience.It is impossible to get the taxi driver to stop （55）.“They're self-confident and free thinking,”said Malcolm Linskey,the author of a history of taxi drivers in London. They are also expensive.London has the most expensive taxis of any city in the world except Tokyo.That's why Ken Livingstone,the Mayor of London,is planning to make taxi drivers negotiate their fares with（56）before they take a ride. Drivers agree that their fares are expensive.That's because their black taxis（57）more than other cars,they say.And the customer is also paying for more driving expertise（专门知识）than anywhere else in the world. Before someone can qualify as a London taxi driver,that person has to pass a test （58）simply as “The Knowledge”.This involves（59）the name and location of every street within six miles of a point in the exact centre of London.The trainee（受训者）must also learn the exact location of every important building within these streets.Finally he or she must be able to use this knowledge to work out the （60）distance between any two destinations within this area. It can（61）up to three years to pass“The Knowledge”.Every day it is possible to see trainee taxi drivers on the streets of London,taking careful notes of popular destinations before tracing the route to their next stop.Cab driving is a job often（62）down in families.Many taxi drivers take their children out in their spare time to memorize（63）they need to know when it is their turn to do “The Knowledge”. London cabbies also have bigger brains.Recent research found that the part of the brain that remembers things was larger and more（64）in cab drivers.They have to fit the whole of London into their heads,so their brains grow bigger.So perhaps it is not（65）that taxi drivers have lots to say. 51.