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
The primary objective of Basic Econometrics is to provide an elementary but comprehensive introduction to the art and science of econometrics.
A．brief
B．precise
C．simplified
D．extensive
单项选择题
Recent studies have identified four major global environmental risks: acid rain, ozone depletion, deforestation and the greenhouse effect.
A．recognized
B．proved
C．got rid of
D．multiplied
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