单项选择题

A high negative ion count is likely to be found ______.

A．near a pond with a water pump
B．close to a slow-flowing river
C．high in some barren mountains
D．by a rotating water sprinkler
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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.
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
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