真空中有两个点电荷，若保持它们之间的距离不变，带电量都减小为原来的一半，则库仑力变为原来的多少倍？
倍
题目分析：根据库仑定律，后来，所以。
--- Yes. It is in the reading material ____ we ____ reading yesterday.
A．that; did
B．that; were
C．when; were
D．when; did
（2013黑龙江哈尔滨）下列物质的用途错误的是
A．食品常充氨气防腐
B．活性炭净水
C．氧气用于医疗急救
D．不锈钢制炊具
倍
，后来
，所以
。
倍，后来，所以。