单项选择题

This selection is primarily concerned with ______.

A．the eye's adaptation to color
B．the properties of colored surfaces
C．the color of colors
D．the effect of changes in color intensity
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单项选择题
During the Middle Ages, productivity of labor ______.
A．was a primary concern of society
B．was hampered by class relationships
C．began to improve over levels reached by the Greeks
D．was in a period of technical progress
单项选择题
A problem that has plagued some fictional Utopias is ______.
A．creation of super-apes
B．the necessity of breeding super-humans
C．the necessity of breeding subhumans
D．the degradation of beasts
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