单选题：Questions are based on the following passage．No one quite kn

Questions are based on the following passage．
No one quite knows how many tunnels have been dug under London. Some of thecity's many underground channels have been subject to the Official Secrets Act. Othersare so old they have fallen into disuse and been forgotten. Nonetheless, anyone walkingthrough central London can reckon that the busy streets around them are mirrored incriss-crossing ( 十字交叉 ) tunnels below. The world's first subterranean ( 地下的 )railway, the London Underground, opened in the city 150 years ago. Last month the firsttunnel for Crossrail, an ambitious 15 billion ($24 billion) new railway project runningacross London, was completed. It lies 40 meters below the surface of the city, a wonder ofengineering. Why does London do so much tunneling?
London has evolved tightly packed, with narrow streets and alleyways spreading outacross it like arteries. Between 1801 and 1851 its population grew from around 960,000 toover 2.5 million. Railway lines terminated at the edges of the city, as it was then defined,at Paddington and Victoria. As visitors and workers flocked to the city, its central roadsbecame increasingly congested. Underground railways were built in 1863 to relieve thechoked streets. Not everyone liked the early metro system, particularly as the trains stillran on sooty ( 煤烟熏黑的 ) steam engines. "I had my first experience of Hades today,"
spluttered R.D. Blumenfeld, an American journalist, in 1887, "and if the real thing is tobe like that I shall never do anything wrong." As the wealthiest city in the world at thetime, London's private investors could afford to sink money into the tunneling schemes.
London's congestion created the need for tunnels and its booming economy providedthe financing. But what made them feasible was the city's location. The clay on whichmost of the city is built provided an excellent tunneling medium. It is soft enough tobe excavated easily, but impermeable ( 防渗透的) enough to stay dry. Once it is dugthrough it will not crumble. It has a "stand-up time", says Roger Bridge of the BritishTunneling Society--when the first Crossrail tunnel was being built, parts of the sectioncould be bored out and then explored as the clay stayed in place. Harder rock wouldrequire more powerful machines or explosives to dig through it. In contrast, cities suchas Dublin are built on a mixture of sandstone and boulder clay. The boulders make itdifficult to mine through, while the sand absorbs water, making it less sturdy. Before thedevelopment of better technology, such as pressure-balance machines, such conditionsmade tunneling tricky. Indeed, parts of south London have fewer tunnels than the centrebecause fewer of its neighborhoods are built on clay.
What do we learn about the London Underground from the passage? A．It was popular among all the citizens at the beginning.
B．It was the first underground railway in the world.
C．It cost twenty-four billion dollars in total.
D．It was opened in the city 510 years ago.