A certain stock exchange designates each stock with a one- , two-,or three-letter code ,where each letter is selected from the 26 letters of the alphabet. If the letter may be reapeated 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?
A 2951
B 8125
C 15600
D 15302
E 18278 | 
