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GWD数学请教
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 alphabetic 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
我这题怎么着算都是A，有同学能给指点一下吗？ |
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