Two spheres having equal mass m, charge q and radius R, are moving towards each other. Both have speed u at an instant when distance between their centers is 4R. Minimum value of u so that they touch each other is
(A) $\sqrt{\frac{q^2}{4\pi\varepsilon_0 mR}}$
(B) $\sqrt{\frac{q^2}{8\pi\varepsilon_0 mR}}$
(C) $\sqrt{\frac{q^2}{16\pi\varepsilon_0 mR}}$
(D) $\sqrt{\frac{q^2}{\pi\varepsilon_0 mR}}$