Which of the following tables represents a function? (PLEASE HELP!)

x 3 3 4 4
y 2 −2 3 −3

x 5 −5 5 −5
y 2 2 8 8

x 3 3 5 5
y 4 5 6 7

x 4 −4 5 −5
y 2 2 3 3