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
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.
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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