Answer: Graph A (upper left corner)
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Step-by-step explanation:
Graph B, C and D all fail the vertical line test. If it's possible to pass a vertical line through more than one point on the curve, then it's said to have failed the vertical line test. Any vertical line itself therefore is automatically not a function. If a graph fails the vertical line test, then it is not a function.
A function is only possible if for any given input x, we have exactly one and only one output. For graph A, the output is the same value regardless of the input. This is a constant function.
For something like graph B, the input x = 0 produces three different y outputs. The output y = 0 is one of those, and the others are off the screen so it's hard to tell what they are. Turns out it doesn't matter and all we need to do is the vertical line test. Graphs C and D are a similar story. In the case of graph D, the only input x allowed is x = 2 and it produces infinitely many y outputs.