A container filled with viscous liquid is moving vertically downwards with constant speed 3${\mathrm{v}}_0$. At the instant shown, a sphere of radius r is moving vertically downwards (in liquid) has speed ${\mathrm{v}}_0$. The coefficient of viscosity is $\mathrm{\eta }$. There is no relative motion between the liquid and the container. Then at the shown instant, the magnitude of viscous force acting on sphere is

• A

  $6{\mathrm{\pi \eta rv}}_0$

• B

  $12{\mathrm{\pi \eta rv}}_0$

• C

  $18{\mathrm{\pi \eta rv}}_0$

• D

  $24{\mathrm{\pi \eta rv}}_0$