正确答案： C

解析：
细节题。录音提到“learner drivers are not allowed to work and earn money as drivers. Therefore, many of them keep their previous jobs”，即在学习驾驶时，没有薪水，故需要“keep previous jobs”。所以答案为C。
【录音原文】
　　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 1 to 4 are based on the conversation you have just heard.
1. Why are London taxi drivers very efficient?
2. How long does the training period last?
3. Why does the speaker think the driving test is a terrible experience?
4. Why do learner drivers have to keep their present jobs?