A certain stock exchange designates each stock with a one-, or two-, or three-letter code, where each letter is selected from the 26 letters of the alphabet. If the letters may be repeated and if the same letters used in a different order constitutes a different code, how many different stocks is it possibly to uniquely designate with these codes?
A. 2951
B. 8125
C. 15600
D. 16302
E. 18278
Answer : E
Please tell me how to do ??? Thankx
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