II. Determine whether each set of ordered pairs represents a function or not.
15. {(4,0), (4,1),(4,2))
16. {(2,0), (-1,1), (-1,3), (4,-2)}
17. {(0,1),(0,2), (0,3), (0,-1), (0,-4) }
18. {(1,0), (-1,0), (-2,0), (3,0), (-4,0) }
19.{(1,2), (-2,1),(-3,1), (-1,3), (4,2)}​