The terminal velocity of a rain drop of radius r falling down in the atmosphere is

• A

  $\frac{2{\mathrm{r}}^2\left(\mathrm{\rho }-{\mathrm{\rho }}_0\right)\mathrm{g}}{9\mathrm{\eta }}$

• B

  $\frac{2{\mathrm{r}}^2\left(\mathrm{\rho }+{\mathrm{\rho }}_0\right)\mathrm{g}}{9\mathrm{\eta }}$

• C

  $\frac{2{\mathrm{r}}^2\left(\mathrm{\rho }-{\mathrm{\rho }}_0\right)\mathrm{g}}{\mathrm{\eta }}$

• D

  $\frac{2{\mathrm{r}}^2\left(\mathrm{\rho }+{\mathrm{\rho }}_0\right)\mathrm{g}}{\mathrm{\eta }}$