Pls ! which relation is a function ?

To be a function every value of x have to correspond only one value of y.
Then means, when we are looking relation, every x- value should be met only one time. It must not to be repeated.
A. x values :5, 4, -3,0 - a function
B. x values : -1, 3,-1,2 (not a function, -1 is repeated two times, for x =-1 we have two values of y)
C. x values: 4,3,-1,4 (not a function, 4 is repeated two times)
D. x values: -4, 3,-4,-3 (not a function, -4 is repeated two times)

A