单项选择题

This passage is about a______.

A．law
B．hypothesis
C．guess
D．theory

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单项选择题

A study of thunderclouds over the North Atlantic showed that lightning occurred only when the air temperature______.

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

单项选择题

Ice particles bouncing off hail falling through a cloud acquire______.

A．a negative charge
B．no electric charge at all
C．a positive charge
D．bothA) and

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