Answer: Choice D
Yes, there exists one output for every input.
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Explanation:
x = input
y = output
For any given x value, we have exactly one and only one y value that pairs up with it. Use the vertical line test. This is where we try to draw a single vertical line through more than one point on the curve. You'll find that no such line can be drawn. Any vertical line only crosses the curve exactly once (corresponding to exactly one output). We say that it passes the vertical line test, and therefore we have a function.
Side note: the curve does not pass the horizontal line test. That means the function is not one-to-one.
