A、The department store Selfridges. B、Cavendish Square. C、Oxford Street. D、Paddington. C

Five years ago, some young thrill-seekers went underground in London. Collectively dubbing themselves “Consolidation Crew”, individually with names like “Urban Fox” and “Silent Motion”, their mission was to “explore everything”. What they did and still do is “place hack”—in other words, trespass on other people’s property.
(16)In 2011, they made “an epic discovery of the highest order”—a “silent nocturnal city”, some 25 meters (80 feet) below the streets of London, where the air was so still that calcium stalactites had formed. It felt, they wrote, “like we were in an Indiana Jones movie—hot, sweaty and dank”.
Urban Fox and company went down a steep iron spiral staircase and into a series of extraordinary miniature man-made tunnels. Just as they had hoped, they’d found the defunct British Post Office underground railway network—the so-called “Mail Train”. (17)The line runs along Oxford Street, under the department store Selfridges and Cavendish Square, and snakes along for 10 kilometers from Paddington in West London to Whitechapel in the East End. For trainspotters, this was a fabulous, almost mythical slice of railway ana—“the least documented railway line this side of the old Iron Curtain”. Only Post Office staff ever went down to these secret subterranean tunnels, where the world’s first driverless electric trains rattled back and forth at speeds of up to 55 kph (35 mph) for most of the 20th century.
(18)Londoners have long been pioneers in building underground, usually out of necessity. The Victorians began it; after the cholera epidemic of 1848-1849 and “The Great Stink” of 1858, an elaborate sewer system was built under the city designed by the “Sewer King”, Joseph Bazalgette.
Questions 16 to 18 are based on the passage you have just heard.
16.How deep was the “silent nocturnal city” located below the streets of London?
17.Where does the line run along?
18.When did Londoners begin to build underground?