Which of these mappings is a function?

A value 5 in x corresponds to 6 in y. A value 6 in x corresponds to 2 in y and 4 in y. A value 7 in x corresponds to 8 in y. A value 8 in x corresponds to 4 in y.
W. A value 5 in x corresponds to 2 in y. A value 6 in x corresponds to 8 in y. A value 7 in x corresponds to 6 in y. A value 8 in x corresponds to 4 in y.
X. A value 5 in X corresponds to 4 in Y. A value 6 in X corresponds to 2 and 6 in Y. A value 7 in X corresponds to 8 in Y. A value 8 in X corresponds to 6 in Y.
Y. A value 5 in X corresponds to 8 in Y. A value 6 in X corresponds to 6 in Y. A value 7 in X corresponds to 2 and 8 in Y. A value 8 in X corresponds to 4 in Y.
Z.
A.
W
B.
X
C.
Y
D.
Z