Question

determine if the set of ordered pairs (x,y) represents y as a function of x (Yes, No)

A) {-4,2 , -2,9}
B) {(-4,2), (2,-2), (-4,9)}
C) {(-4,2), (2,-4), (9,9)}
D) {(-4,2), (-2,9), (-2,-4), (9,-2)}

Answer: All No except C, C is yes

Please explain why A, B, and D are no and C is yes