In many big cities, there are usually more than one underground line(地铁线). The fastest way 1 in a city is by subway. How to travel by subway? Do you know? Here is some information 2 before you travel by subway. First, you should 3 the right line. If you don't, you will have to 4 and take another one. Second, you need some coins 5 your ticket, because 6 the machines which sell tickets only accept coins. Coins can be changed next to the ticket machines. Or you can go to the ticket office 7some. 8 you have a prepaid card, you can just go ahead. After you have bought the 9, you should wait for your subway outside the yellow line. You 10 walk too close to the yellow line. It's dangerous.