A linear function f(x) contains the coordinates (-9, 4) and (6,4). Joy says f(x) is a horizontal line, and Lu says f(x) is a vertical line. Which student correctly described the function? Explain. A. Joy because (-9,4) and (6,4) would have a slope of zero which is a horizontal line.

B. Joy because (-9,4) and (6,4) would have a slope of zero which is a vertical line.

C. Lu because (-9,4) and (6,4) would have a slope of zero which is a horizontal line.

D. Lu because (-9,4) and (6,4) would have a slope of zero which is a vertical line.