A:大雁南飞
B:骆驼刺根系发达
C:海豹皮下脂肪厚
D:千里之堤,溃于蚁穴
Helen didn't know anything about it her father told her .
两个完全相同的金属球A和B带电量之比为1:7 ,相距为r 。两者接触一下放回原来的位置,若两电荷原来带异种电荷,则后来两小球之间的静电力大小与原来之比是( )