which of the following ordered pairs is not a function?

Question

a) { (0,0), (2,1), (3,5), (6,4) }

b) { (-3,-1), (3,0), (4,2), (5,7) }

c) { (-2,9), (-1,8), (2,6), (6,8) }

d) { (4,5), (6,9), (10,3), (12,0) }

I think its (A) but im not quite sure..

Answer 1

all of them are functions. They all pass the vertical line test. There's only 1 y value for each x value

C) though is not a "one to one function" as it fails the horizontal line test. there are two x values that have the same y value. due to the points (-1,8) and (6,8)