Which of the following expressions represents a function? a. x^2 + y^2 = 9 b. {(4, 2), (4, −2), (9, 3), (9, −3)} c. x = 4 d. 2x + y = 5

A. x^2 + y^2 = 9: Equation of a circle: Thus is not a function

B. {(4, 2), (4, −2), (9, 3), (9, −3)}: This is not a function, since we have for the same INPUT 2 different OUTPUT 

C. x = 4. This is a linear function that is // to y-axis

D. 2x + y = 5 . This is a linear function