Passwords in a computer installation take the form X₁, X₂, X₃, X₄, where each character X_i  is one of the 26 letters of the alphabet. Determine the maximum possible number of different passwords available for assignment for each of the two following conditions:

(a) A given letter of the alphabet can be used only once in a password.
(b) Letters can be repeated if desired, so that each X_i is completely arbitrary.
(c) If selection of letters for a given password is completely random, what is the probability that your competitor could access, on a single try, your computer in part (a)? part (b)?