Which of the relations given by the following sets of ordered pairs is a function?
{(5, 4), (4, 3), (3, 2), (4, 5), (2, 1)}
{(6, 1), (3, 1), (3, 5), (2, 4), (1, 2)}. {(1,2), (-4, 8), (- 3, 5), (1, - 2), (7, 12)}
{(5, 4), (5, 6), (5, 8), (5, 10), (5, 12)}