单选题：Questionsare based on the following passage. Gulliver has a

Questionsare based on the following passage.
Gulliver has a friend who recently gave up his job to study for "The Knowledge", the notoriously difficult programme that London's black-taxi drivers must pass through before getting a license.
Would-be cabbies must first gain an encyclopedic knowledge of the capital's central district--some 25,000 streets and approximately 20,000 landmarks--and be able to recite the best way of navigating between them．Studying for The Knowledge typically takes from two to four years.
The sacrifice used to be worth it．Although driving a taxi for a living is undoubtedly hard work, cabbies earn a decent wage, choose their own hours and usually manage to spend a few weeks a year at their holiday villas in Spain．Alas, for Gulliver's friend, it is no longer a job with prospects．Leaving aside that learning routes by rote in an era of satellite navigation is a waste of everyone's time, the reason that cabbies put themselves through such a tough selection process is so they can earn the privilege of picking up passengers off the street, which only they are legally allowed to do.
Uber is malting this privilege increasingly irrelevant．The firm uses a smartphone platform to bring passengers and drivers together．It is on its way to cornering the world taxi market--although, like many cabbies, it is taldng a circuitous ( 迂回的 ) route．Several American cities, including Portland, have ordered the firm to suspend operations, while whole countries, such as Germany, have outlawed it．Still, these are mere bumps in the road．The latest report by Certify, which tracks business- expense claims, found that for the first time the majority of "ground transportation receipts" were for rides in Uber cars．In the second quarter of 2015, 55% of such business expenses emanated from that single company, compared with 43% on all other taxi services．According to Certify, whose respondents are overwhelmingly American, the cities in which businessmen are most likely to use an Uber car are San Francisco ( 79% ), followed by Dallas (60%) and Los Angeles (54%)．It is easy to see why．Uber is cheap, reliable and easy to use．You know which driver is coming for you and the driver knows you．There is no need to play a game of hailing leapfrog ( 交替进行 ) with competitors along busy streets, in the hope of finding a taxi with a light on.
On a recent trip to New York, Gulliver's young daughter was desperate to take a ride in a yellow taxi because she had seen them on posters．So we took a ride as a tourist attraction．When it came to pulling our suitcases back to JFK, though, it was much more convenient to call an Uber car．Gulliver worries for his friend's choice of new career．How long will it be before he becomes little more than a curiosity for those wanting to experience ye olde England?
What do we know about "The Knowledge"? A．It is an encyclopedia about London streets.
B．It is an exam would-be cabbies must pass.
C．It is a navigation between the landmarks.
D．It is a license for the cabby profession.