Which set of ordered pairs in the form of (x, y) does not represent a function of x?
a. {(1, 1.5), (2, 1.5), (3, 1.5), (4, 1.5)}
b. {(0, 1.5), (3, 2.5), (1, 3.3), (1, 4.5)}
c. {(1, 1.5), (–1, 1.5), (2, 2.5), (–2, 2.5)}
d. {(1, 1.5), (–1, –1.5), (2, 2.5), (–2, 2.5)}