In order for a function...to be called a function , is that for every x, there must always only be one y. Two x's can have the same y, but the same x can't have two separate y's. One way to test this is the vertical line test. You basically put up a vertical line on the graph. The artificial line (you can do it with your finger, pencil, etc.) should only intersect once on the graph. So, take your pencil, and hold it up to down (like this | ) on your monitor. You see that when you do that, it intersects with the graph twice. That means it DOESN'T pass the vertical line test, which means it is not a function (Function are not always straight lines, which is why the first two are wrong). So, the answer is
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