Let A, B and C be three events such that the probability that exactly one of A and B occurs is (1 k), the probability that exactly one of B and C occurs is (1 2k), the probability that exactly one of C and A occurs is (1 k) and the probability of all A, B and C occur simultaneously is k², where 0 < k < 1. Then the probability that at least one of A, B and C occur is :