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
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
单项选择题
In the flint decades of the twentieth century, the individual gem could not be seen, but could be worked with fruitfully.
A．blindly
B．completely
C．productively
D．carefully
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