How does the opponent-process theory explain the phenomenon of afterimages, such as seeing a blue afterimage when staring at a yellow object?

Afterimages are the result of the primary color's signal being strengthened over time.

Afterimages appear because the visual system temporarily enhances the perception of the primary color's complementary color.

Afterimages occur as the visual system fatigues from processing the primary color and enhances the complementary color.

Afterimages are unrelated to the opponent-process theory and are caused by external factors.