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

A. 4

B. 5

C. 6

D. 7

E. 8

A B C D E.....中选1个或2个排12个不同的号码。最少需要几个字母

如果用4个字母，P(4,2)/2+p(4,1)=10不够

5个字母。P(5,2)/2+p(5,1)=10+5=15.

可以验证一下： 4个字母时：A. B . C .D. AB. AC. AD. BC. BD. CD

5个字母：A. B. C . D. E. AB. AC AD AE BC BD BE CD CE DE (记住要按字母前后顺序，和只选1和2个)

x+x(x-1)/2>12,求出是要大于5。之所以要除以2是in alphabetical order，呵呵，我自己做的时候没注意，汗啊。