Which relations are functions?
select yes or no for each relation.
(2, 2), (3, 3), (4, 4), and (5, ,no)
(1, 2), (1, 3), (1, 4), and (1, ,no)
(2, 1), (3, 1), (4, 1), and (5, ,no)
(2, 2), (2, 3), (5, 4), and (5, ,no)