Cells in the parvo system can distinguish between two colors at any relative brightness of the two. Cells in the color-blind magno system, on the other hand, are analogous to a black-and-white photograph in the way they function: they signal. information about the brightness of surfaces but not about their colors. For any pair of colors there is a particular brightness ratio at which two colors, for example red and green, will appear as the same shade of gray in a black-and-white photograph, hence any border between them will vanish. Similarly at some relative red-to-green brightness level, the red and green will appear identical to the magno system. The red and green are then called equiluminant. A border between two equiluminant colors has color contrast but no luminance contrast The author mentions a "black-and-white photograph" most probably in order to explain

A. how the parvo system distinguishes between different shapes and colors.
B. how the magno system uses luminosity to identify borders between objects.
C. the mechanism that makes the magno system color-blind.
D. why the magno system is capable of perceiving moving images.
E. the brightness ratio at which colors become indistinguishable to the parvo system.