A Small Charged Plastic Ball Is Vertically Above Another Charged $\sqrt { 2 }$

Multiple Choice

A small charged plastic ball is vertically above another charged small ball in a frictionless test tube as shown in the figure. The balls are in equilibrium a distance d apart. If the charge on each ball is doubled, the equilibrium distance between the balls in the test tube would become

A) $\sqrt { 2 }$ d.
B) 2d.
C) 4d.
D) 8d.
E) d/4.