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 an comprehensive introduction to the art and science of econometrics.
A．brief
B．precise
C．simplified
D．extensive
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