What is the domain of the following function represented by ordered pairs? {(4, 9), (–6, 1), (–2, 5), (3, 1)}

A. {5, 9}
B.{1, 5, 9}
C.{–6, –2, 3, 4}
D.{–6, –2, 1, 3, 4, 5, 9}