In order for the function to be one-to-one, it must pass the vertical line test AND the horizontal line test.
The vertical line test is where you draw a vertical line anywhere and the line will not intersect the graph more than one time. In other words, the function cannot contain a duplicate x-value.
The horizontal line test is where you draw a horizontal line anywhere and the line will not intersect the graph more than one time. In other words, the function cannot contain a duplicate y-value.
The graph provided passes the vertical line test but fails the horizontal line test. <em>One example is that there are two x-intercepts.</em>
The answer is the last choice. (a, b), in this question, means the domain or range is from a to b, exclusive. When you use ∞, you would always use the exclusive.
