Tell whether the following is a function or not function.
1. f(x) = {(1,2), (2,3), (3,5), (4,7)}
2. g(x) = {(1,3), (1,4), (2,5), (2,6)}
3. ℎ(x) = {(1,3), (2,6), (2,0)}
4. f(x) = [{1, 4}, (2, 4}, {−2, 4}, {−1, 4}]