Which relation is a function?
•{(4, 2), (3, 3), (2, 4), (3, 2)}
•{(1, 4), (2, 3), (3, 2), (4, 1)}
•{(1, -1), (-2, 2), (-1, 2), (1, - 2)}
• {(1, 2), (2, 3), (3, 2), (2, 1)}​