answer: #Person1# tells #Person2# the bus system crashed. #Person4# explains to #Person3# that's because everyone checked the balance of their IC cards.
documents: #Person1#: Hey, do you know the bus company network system crashed? 
  #Person2#: I heard about it several days ago! 
  #Person1#: That is unbelievable! 
  #Person3#: Daddy, can you tell me what they are talking about? 
  #Person4#: They are talking about the network of the bus company. 
  #Person3#: Why is their network not working? 
  #Person4#: Because of the IC cards. 
  #Person3#: The IC card? 
  #Person4#: Yes, Beijing is a big city and there are about 7, 000, 000 people using IC cards on buses. 
  #Person3#: And? 
  #Person4#: All the people were checking the balance of their cards on the Internet at the same time, that's why the system crashed. 
  #Person3#: Oh, that is horrible!