A solid rubber ball of density p and radius Ii falls vertically through air. Assuming that the air resistance on the ball is F = I .R v, where K is a constant and y is the 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

  $\frac{4{\mathrm{\pi \rho gR}}^2}{3\mathrm{K}}$

• B

  $\frac{4\mathrm{\pi R\rho g}}{\mathrm{K}}$

• C

  $\frac{4\mathrm{\rho g}}{3{\mathrm{KR}}^2}$

• D

  $\frac{4{\mathrm{\rho gR}}^3}{3\mathrm{K}}$