A、They look forward to further promotion. B、They want to earn money from both jobs. C、They cannot earn money as taxi drivers yet

London Taxi drivers know the Capital like the back of their hands. No matter how small and instinct(模糊的)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 have all gone through a very tough training period to get a special taxi-driving license. During this period which can take from 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 motor bikes practising how to move to and from different points of the city. Learner taxi drivers are tested several times during their training period by government officers. Their exams are a terrible experience. The officers ask you: "How do you get from Buckingham 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 would 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 obtain their licenses. The training can cost quite a lot because learners have to pay for their own expenses on the tests and medical exam.
Q:
19. How long does the training period last?
20. Why do learner drivers have to keep their present jobs?