Let A = {p, q, r, s} and B = {1, 2, 3}. Which of the following relations from A to B is not a function?
i. R1 = {(p, 1), (q, 2), (r, 1), (s, 2)}
ii. R2 = {(p, 1), (q, 1), (r, 1), (s, 1)}
iii. R3 = {(p, 1), (q, 2), (p, 2), (s, 3)}
iv. R4 = {(p, 2), (q, 3), (r, 2), (s, 2)}