Which of the following sets represents a function?
{(3,5), (-1, 7), (3, 9))
{(1, 2), (3, 2), (5, 7))
{(1, 2), (1, 4), (1, 6)}

B. {(1, 2), (3, 2), (5, 7))

Explanation – Set B represents a function because all the first elements are different. While in the rest of the given sets, the first elements are repeated. In set A, 3 is repeated twice in the first element and in set C, 1 is repeated all the tree times in the first element.

While drawing it in a graph, one line may cross the other line which says that the given set is not a function. Such type of test is called as vertical line test.