Which of the following expressions represents a function?

x2 + y2 = 9
{(4, 2), (4, –2), (9, 3), (9, –3)}
x = 4
2x + y = 5

A. Not a function because it's a circle that doesn't pass the vertical line test

B. Not a function because we have x = 4 repeated more than once. Same for x = 9 as well.

C. Not a function. Any vertical line fails the vertical line test. Any vertical line is therefore not a function.

D. This is a function since it passes the vertical line test.



In summary, the final answer is choice D