单项选择题

What effect does excessive positive ions have on some people

A．They think they are mad.
B．They feel very tired.
C．They become violently sick.
D．They are too tired to do-anything.
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单项选择题
People should be able to come to a decision about ions in the air if they ______.
A．note their own reactions
B．move to a healthier area
C．observe domestic animals
D．watch how wealthy people behave
单项选择题
By observing the behavior of animals, scientists may be able to ______.
A．prevent disasters
B．organize groups of people
C．predict earthquakes
D．control areas of California
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