Which of the following relations is a function?
{(5, –7), (4, 6), (–3, 8), (5, 9)}
{(8, –4), (–4, 8), (–4, –8), (–8, 4)}
{(2, 3), (–2, 3), (3, 2), (–3, –2)}
{(9, –1), (–1, 9), (9, 2), (2, –1)}