单项选择题

After Benjamin Franklin's discovery, others showed that _______.

A．the tops of thunderclouds have a great positive charge
B．the bottoms of the clouds have a great negative charge
C．only the middles of thunders are electrically charged
D．both A and B
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单项选择题
According to the article, Benjamin Franklin was the first to prove that _______.
A．lightening is caused by hail falling through super - cooled clouds
B．thunderclouds are not charged with electricity
C．thunderclouds are charged with electricity
D．lightening was not an invention of the devil
单项选择题
What is implied in this passage
A．Positive charges are in the top of the cloud, negative charges in the bottom.
B．When positive and negative charges within a cloud cannot break down the insulating properties of air, no lightening will be formed.
C．Without hail and thunder, there could be no lightening.
D．Without refrigerated rods, lightening could not be created.
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