Which of the following relations is NOT a function?
A.(3,5)(4,9)(2,8)(1,7)
B.(4,7)(5,9)(4,9)(2,8)
C.(3,5)(4,5)(2,5)(1,5)

A function is a set of ordered pairs no two points
of which have the same x-coordinate.  (Note, the
y-coordinates of two points can be the same, but
the x-coordinates cannot be.)

The correct choice is B because it contains two points
(4,7) and (4,9) both of which have the same x-coordinate.

Edwin