A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order.  What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

A.       4

B.       5

C.      6

D.      7

E.       8

答案是B。怎么做的啊？ 谢谢高手指点了！！

C（x,2）+x >= 24

(因为按字母顺序排列，所以选两个字母只能有一种排法，相当于组合)

x(x-1)/2 + x >=24

x(x+1)>=24

当x＝4时 左边为20大于等于号不成立，x＝5时左边为30够了，所以是5