Two solid spheres of radii $2 \ mm$ and $4 \ mm$ are tied to the two ends of a light string and released in a liquid of specific gravity $1.3$ and coefficient of viscosity $1 \ Pa \cdot s$. The string is just taut when the two spheres are completely submerged in the liquid. If the density of the materials of the two spheres is $2800 \ kg \cdot m^{-3}$,then the terminal velocity of the system of the spheres is (take $g = 10 \ m \cdot s^{-2}$):

• A
  $2 \ cm \cdot s^{-1}$
• B
  $4 \ cm \cdot s^{-1}$
• C
  $4 \ m \cdot s^{-1}$
• D
  $2 \ m \cdot s^{-1}$