A certain stock exchange designates each stock with a one- , two-,or three-letter code ,where each letter is selected from the 26 letters of the alphabet. If the letter may be reapeated and if the same letters used in a different order constitute a different code , how many different stocks is it possible to uniquely designate with these codes?

A 2951

B 8125

C 15600

D 15302

E 18278

用1个字母，可以表示26个股票

用两个字母，可以表示26*26个股票（字母可以重复）

三个，26*26*26

三个结果相加26 + 26^2 + 26^3

强力硬算结果为E，时间不够的话看最后一位，肯定是8，选E