A safe box requires a four-digit number to be opened.

(i) Find the number n of different numbers that are possible codes for the safe box.

(ii) If a burglar chooses at random four-digit numbers trying to open the box, find the probability that he succeeds

(a) at the kth attempt (1 ≤ k < n);

(b) during one of his first r attempts (1 ≤ r < n);

(c) at the nth (and final) attempt.