WeBWorK using host: https://webwork-ptx.aimath.org, format: simple seed: 378

The graph of a non-invertible function [math] is shown below.
Also shown is a horizontal line which you can click and drag to different heights. If that line intersects the graph of the function [math] the point(s) of intersection will be shown.
Next, the graph of an invertible function [math] is shown.

According to these graphs, what is true about the graph of an invertible function?