GWD求助，偶觉得这道题的答案都不对~~

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?

答案选5哈

假设X个字母的话，数量不是X+C(X,2)P(2,2)吗~

谢谢

我总是想着算出来要等于12，嗨，谢谢：）

我以为你问这个题目呢：

In the figure shown, the circle has center O and radius 50, and point P has coordinates (50,0).  If point Q (not shown) is on the circle, what is the length of line segment PQ ?

(1)     The x-coordinate of point Q is – 30.

(2)     The y-coordinate of point Q is – 40.