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An interactive three dimensional plot is shown with a dropdown box giving the options “Critical Point 1”, “Critical Point 2”, and “Critical Point 3”. For the Critical Point 1 option, a saddle surface is shown with a point highlighted at middle of the saddle. For the Critical Point 2 option, an upward curving surface is shown with a point highlighted at the bottom of the surface. For the Critical Point 3 option, an upward curving surface is shown with a point highlighted at the bottom of the surface.