Question R1 Q3 - NSB 2013

Which of the following relations does NOT represent a function?

W) {(2, 3), (-4, 3), (7, 3)}

X) {(0, 0), (-1, -1), (2, 2)}

Y) {(2, 3), (-5, 3), (2, 7)}

Z) {(-1, 3), (-5, 3), (-9, 0)}