请教prep的几题

40.  

Beth's bank charges a service fee on a regular checking account for each month in which the balance on the account falls below $100 at any time during the month.  Did the bank charge a service fee on Beth's regular checking account last month?

(1)  During last month, a total of $1,000 was withdrawn from Beth's regular checking account.

(2)  At the beginning of last month, Beth's regular checking account balance was $500.

 我选择E...reason:不知道在这个时间段内账户余额发生什么样的变化

 我选择E...reason:不知道在这个时间段内账户余额发生什么样的变化

(1) For each tank, 30 percent of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.

(2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

这两道题都有点搞不明白在说什么。。。。

 这道题目A，方法参考标准方差的算法

41.   5676-!-item-!-187;#058&004686

All of the stocks on the over-the-counter market are designated by either a 4-letter or a 5-letter code that is created by using the 26 letters of the alphabet.  Which of the following gives the maximum number of different stocks that can be designated with these codes?

(A) 2(26^5)

(B) 26(26^4)

(C) 27(26^4)

(D) 26(26^5)

(E) 27(26^5)

这个题 我选E. 是不是有哪里没有理解到位啊？