题目:
A certain stock exchange designates each stock with a one-, two-, or three-letter code, where each letter is selected from the26 letters of the alphabet. If the letters may be repeated 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?
选项:
A、 2,951
B、 8,125
C、 15,600
D、 16,302
E、 18,278
答案:
E