单项选择题

A study of thunderclouds over the North Atlantic showed that lightening occurred only when the air temperature ______.

A．around the cloud was below freezing
B．above the cloud was lower than the temperature below the cloud
C．around the cloud was above freezing
D．of the cloud was above freezing
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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
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
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