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

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热门试题

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

单项选择题

The present selection has apparently been preceded by some explanation of ______.

A．some experiments with color pigments
B．the nature of color
C．the color properties of various surfaces
D．the mechanism of the eye's adaptation to color