A、Because London taxi drivers all have gone through a very tough training period to get special taxi driving license. B、Because

London taxi drivers know the capital like the back of their hands. No matter how small or indistinct the street is, the driver will be able to get you there without any trouble. The reason London taxi drivers are so efficient is that they all have gone through a very tough training period to get special taxi driving license. During this period, which can take two to four years, the would-be taxi driver has to learn the most direct route to every single road and to every important building in London. To achieve this, most learners go around the city on small motorbikes practicing how to move to and from different points of the city. Learner taxi drivers are tested several times during the training period by government officers. The exams are terrible experience. The officers ask you "How do you get from Birmingham Palace to the Tower of London?" and you have to take them there in the direct line. When you get to the tower, they won’t say "well done". They will quickly move on to the next question. After five or six questions, they will just say "See you in two months’ time." and then you know the exam is over. Learner drivers are not allowed to work and earn money as drivers. Therefore, many of them keep their previous jobs until they have obtained the license. The training can cost quite a lot, because learners have to pay for their own expenses on the tests and the medical exam.
Questions 16 to 19 are based on the passage you have just heard.
16. Why it says that London taxi drivers know the capital like the back of their hands?
17. How long will a would-be driver get special taxi driving license?
18. What does the would-be taxi driver has to learn during the training period?
19. Why it says the training cost is quite a lot?