Radio stations in a certain country use a sequence of 4 or 5 letters as their station identification call letters. The first letter must be X or K. Assume there are no restrictions on the remaining​ letters, and repetition is allowed. ​a) How many 4​-letter station identifications are​ possible? ​b) How many 5​-letter station identifications are​ possible? ​c) How many total station identifications are​ possible? ​d) The identification for a​ randomly-chosen radio station is 4 letters in length. What is the probability that all four letters are​ different?