At the beginning of the 20th century, many people thought that the family was falling apart—__________, they thought it was dying. ( )
A:on the other hand
B:for another
C:in other words
D:in addition
在一半径为R的圆周上均匀分布有N个带电小球(可视为质点)无间隙排列,其中A点的小球带电荷量为+3q,其余小球带电荷量为+q,此时圆心O点的电场强度大小为E,现仅撤去A点的小球,则O点的电场强度大小为( )