Which of the relations given by the following sets of ordered pairs is a function?
{(1, 2), (2, 3), (3, 4), (3, 3), (2, 2), (1, 1)}
{(1, 1), (1, 3), (1, 5), (1, 7), (1, 9)}
{(2, -7), (6, 5), ( − 3, 9), (2, 0), ( − 5, 3)}
{(2, 1), (7, 5), ( − 4, 0), (3, 1), (0, −6)}