If A = {1, 2, 3 }, B = {4, 5, 6 }, which of the following is not a relation from A to B
R3 = {(1, 4), (1, 5), (3, 6), (2, 6), (3, 4)}
R1 = {(1, 4), (1, 5),(1, 6)}
R2 = {(1, 5), (2, 4), (3, 6)}
R4 = {(4, 2), (2, 6), (5, 1), (2, 4)}
R4 is a set of ordered pairs where each first element does not come from A.