单项选择题

Recent research into aging suggests that the body's defense mechanisms may lose the ability to distinguish what is alien.

A．insane
B．infectious
C．foreign
D．poisonous
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单项选择题
Both a person's heredity and his environment help to shape his character.
A．relative
B．education
C．nationality
D．surroundings
单项选择题
A number of animals in Aesop's fables are portrayed as being crafty.
A．vain
B．virtuous
C．artistic
D．cunning
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