题目内容：
根据以下内容回答题,Banks are not ordinarily prepared to pay out all accounts; they rely on depositors (储户) not to demand payment all at the same time. If depositors should come to fear that a bank is not safe, that it cannot pay off all its depositors, then that fear might cause all the depositors to appear on the same day. If they did, the bank could not pay all accounts. However, if they did not all appear at once, then there would always be enough money to pay those who wanted their money when they wanted it. Mrs. Elsie Vaught has told us of a terrifying bank run that she experienced． One day in December of 1925 several banks failed to open in a city where Mrs. Vaught lived． The other banks expected a run the next day, and so the officers of the bank in which Mrs Vaught worked as a teller had enough money on hand to pay off their depositors. The officers simply told the tellers to pay on demand． The next morning a crowd gathered in the bank and on the sidewalk outside. The length of the line made many think that the bank could not possibly pay off everyone. People began to push and then to fight for places near the tellers' windows, The power of the panic atmosphere was such that two tellers, though they knew that the bank was quite all right and could pay all depositors, drew their own money from the bank. Mrs Vaught says that she had difficulty keeping herself from doing the same.
A bank run happens when _______. A．a bank is closed for one or more days
B．too many depositors try to draw out their money at one time
C．there is not enough money to pay all its depositors at one time
D．tellers of a bank take their own money from the bank
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