Which of the following sets represents the domain of the function shown? (5 points) {(2, 7}, (15, 13), (21, –4), (24, 18)}

a
{–4, 2, 7, 13, 15, 18, 21, 24}

b
{(7, 2), (13, 15), (–4, 21), (18, 24)}

c
{2, 15, 21, 24}

d
{–4, 7, 13, 18}