(i){P, R, I, N, C, A, L}
(ii){0}
(iii){1, 2, 3, 6, 9, 18}
(iv){3, -3}
(a) {x : x is a positive integer and is a divisor of 18}
(b) {x : x is an integer and x2 - 9 = 0}
(c) {x : x is an integer and x + 1 = 1}
(d) {x : x is a letter of the word PRINCIPAL}