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补全短文 

Lightning is generated when electrical charges separate in rain clouds， though processes are still not fully understood. Typically， positive charges（正电荷） build at the cloud top， while the bottom becomes negatively charged（负电荷的）。 In most instances of cloud-to-ground lightning， the negatively charged lower portion of the cloud repels negatively charged particles on the ground‘s surfaces， making it become positively charged. The positive charge on the ground gathers at elevated points.

In most cases of cloud-to-ground lightning， the ground‘s surface__________.

A.is positively charged

B.is shifted sideways by strong winds

C.is negative charged

D.is changed from positively charge to negative charge

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