求助大神帮我走出PREP07 66 的牛角尖啊太难受了！

66.        (35089-!-item-!-188;#058&007596)

Bank depositors in the United States are all financially protected against bank failure because the government insures all individuals' bank deposits.  An economist argues that this insurance is partly responsible for the high rate of bank failures, since it removes from depositors any financial incentive to find out whether the bank that holds their money is secure against failure.  If depositors were more selective, then banks would need to be secure in order to compete for depositors' money.

The economist's argument makes which of the following assumptions?

(A) Bank failures are caused when big borrowers default on loan repayments.

(B) A significant proportion of depositors maintain accounts at several different banks.

(C) The more a depositor has to deposit, the more careful he or she tends to be in selecting a bank.

(D) The difference in the interest rates paid to depositors by different banks is not a significant factor in bank failures.

(E) Potential depositors are able to determine which banks are secure against failure.

非常理解E的正确选项，但是不知道为什么在浑噩中选了B以后虽然知道B不对是无关但就是说服不了自己。求哪位大神帮我捋一下思路详细讲解一下B错在哪里啊。。。
谢谢！