$A$ spherical solid ball of volume $V$ is made of a material of density $\rho_1$. It is falling through a liquid of density $\rho_2$ $(\rho_2 < \rho_1)$. Assume that the liquid applies a viscous force on the ball that is proportional to the square of its speed $v$,i.e.,$F_{viscous} = -kv^2$ $(k > 0)$. The terminal speed of the ball is:

• A
  $\frac{Vg(\rho_1 - \rho_2)}{k}$
• B
  $\sqrt{\frac{Vg(\rho_1 - \rho_2)}{k}}$
• C
  $\frac{Vg\rho_1}{k}$
• D
  $\sqrt{\frac{Vg\rho_1}{k}}$