单项选择题
A.affects the performance of workers in restaurants. B.influences animal feeding habits. C.makes animals like hamsters more active. D.prepares humans for having greater leisure time.
A.They were less able to concentrate. B.Their body temperature fell too quickly. C.Their mental functions were immediately affected by the cold. D.They were used to swimming pool conditions.
A.were unusually aggressive and cruel. B.had had their way of life destroyed by invaders. C.were an extremely primitive society. D.had only recently made permanent settlements.