A solid rubber ball of density d and radius E falls vertically through air. Assume that the air resistance acting on the ball is F = kRV, where k is constant and V its velocity. Because of this air resistance, the ball attains a constant velocity called terminal velocity V_t after some time. Then V_t is

• A

  $\frac{4{\mathrm{\pi R}}^2\mathrm{dg}}{3\mathrm{k}}$

• B

  $\frac{4{\mathrm{\pi Rd}}^2\mathrm{g}}{3\mathrm{k}}$

• C

  $\frac{4{\mathrm{\pi Rdg}}^2}{3\mathrm{k}}$

• D

  $\frac{4\mathrm{\pi Rdg}}{3\mathrm{k}}$