听第10段材料，回答题。
How many tube lines are there in London?

Text 10
In today' s program, we are talking about London Underground trains．
The quickest way to get around London is no doubt ' the tube', London' s way of saying the Underground train． The tube is made up of twelve lines,each bearing a name and a color on the tube map． For example, the Central Line is red． And as its name suggests, it runs from east to west, taking in central London． The Circle Line is yellow, and is the only line which runs in a complete circle． About 2．5 million people use the tube each day． If you want to travel fast around London, taking the tube will get you to wherever you want quicker than bus or taxi． Trains generally leave every 3 or 4 mi-nutes, less frequently at weekends and late at night． There are maps at each station, as well as inside each train, to make sure that you can see where you are going and that you are on the right train． If you find yourself going in the wrong direction, simply get off at the next station and cross the plat-form to the other side． Every train will show its direction on the front of the train． So, it is a good idea to look at the train as it is coming into the sta-tion．