Determine whether each relation is a function.
1. {(3, -8), (3, 2), (6, -1), (2, 2)}
2. {(0, 1), (-4, -3), (-3, 6), (3, 6)}
3. {(-6, 3), (2, -2), (0, 8), (1, 1)}
4. {(1, 8), (-6, 21), (-11, 21), (-3, 11), (0, 21)}