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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.

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Isadora Duncan’s masterly writings on the dance reveal the depth of her determination to create a lyric form of the art which was free of characterization, storytelling, and the theatrical exhibition of skills. She wished to discard the traditional methods and established vocabularies of such dance forms as ballet and to explore the internal sources of human expressiveness. She shunned bodily ornamentation and strove to use only the natural movements of her body, undistorted by acrobatic exaggeration and stimulated only by internal compulsion. In her recitals Duncan danced to the music of Beethoven, Wagner, and Gluck, among others, but, contrary to popular belief, she made no attempt to visualize or to interpret the music; rather, she simply relied on it to provide the inspiration for expressing inner feelings through movement. She did not regard this use of music as ideal, however, believing that she would someday dispense with music entirely. That day never came. The author implies that Duncan relied on music in her recitals in order to

A. interpret musical works solely by means of natural body movements.
B. foster the illusion that music serves as an inspiration for the dance.
C. inspire the expression of inner feeling when she danced.
D. validate the public belief that music inspires the expression of feeling through movement.
E. counter the public belief that she made no attempt to visualize music.

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The correlation between carbon dioxide and temperature throughout the Vostok record is consistent and predictable. The absolute temperature changes, however, are from 5 to 14 times greater than would be expected on the basis of carbon dioxide’s own ability to absorb infrared radiation, or radiant heat. This reaction suggests that, quite aside from changes in heat-trapping gases, commonly known as greenhouse gases, certain positive feedbacks are also amplifying the temperature change. Such feedbacks might involve ice on land and sea, clouds, or water vapor, which also absorb radiant heat. The author mentions certain positive feedbacks in order to indicate that

A. increased concentration of carbon dioxide in the Earth’s atmosphere is responsible for global temperature increase.
B. some climate simulation models have produced useful information.
C. greenhouse gases alone do not account for global temperature increase.
D. variables that benefit life are causing global temperature to increase.
E. beneficial substances that are not heat-trapping gases and that contribute to global temperature increase have been found in the Vostok ice core.