The definition of a function is that every x value (within the interval; for continuous functions) has one y value. This means that one x value CANNOT have two y values and still be a function. You can use the vertical line test, do this by imagining a vertical line going up and down the graph. If that line touches two or more points, then that means there are more than 1 y value for at least one of the x values. That is a quick way to determine if it's a function. Only #1 passes this test.
