A Small Charged Plastic Ball Is Vertically Above Another Charged $d$

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