Which of the tables represents a function?

Table A:
Input Output
4 3
3 2
3 1

Table B:
Input Output
1 3
3 5
5 7

Table C:
Input Output
0 0
1 4
1 5

Table D:
Input Output
4 1
4 2
4 3


a) Table A
b) Table B
c) Table C
d) Table D