单选题
[完成句子]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
正确答案：A
答案解析：本段文字解释了云层与地面之间的闪电的成因，云层下部带负电荷，排斥地面表层的负电荷，所以地面表层聚集的是正电荷。答案为A。