Which of the following sets of ordered pairs represents a function?
a.{(0,2), (1,3), (2, 4), (3,5),(4,6) }
b.{(0,1),(0,2), (0, 3), (0,4),(0,5)}
c.{(0,2), (1,3), (1,4), (3,5), (3,6)}
d.{(0,0), (4,2), (4,-2), (9,3),(9,-3) }​