Which of the following sets represents the domain of the function shown?
{(2, 7}, (15, 13), (21,-4), (24, 18). {-4, 2, 7, 13, 15, 18, 21, 24}
{(7, 2), (13, 15), (-4, 21), (18, 24)}
{2, 15, 21, 24}
{-4, 7, 13, 18}