单项选择题

Chimpanzees are frequently used as stand-ins for human beings in experiment.

A．partners
B．role-models
C．stand-bys
D．substitute
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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
Recent research into aging suggests that the body' s defense mechanisms may lose the abili ty to distinguish what is alien.
A．insane
B．infectious
C．foreign
D．poisonous
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