A small ball of mass $M$ and density $\rho$ is dropped in a viscous liquid of density $\rho_0$. After some time, the ball falls with a constant velocity. What is the viscous force on the ball?\\
(1) $F = Mg\left(1 + \frac{\rho_0}{\rho}\right)$\\
(2) $F = Mg\left(1 + \frac{\rho}{\rho_0}\right)$\\
(3) $F = Mg\left(1 - \frac{\rho_0}{\rho}\right)$\\
(4) $F = Mg\left(1 \pm \rho\rho_0\right)$