$A$ small metal sphere of radius $a$ is falling with a velocity $v$ through a vertical column of a viscous liquid. If the coefficient of viscosity of the liquid is $\eta$,then the sphere encounters an opposing force of

• A
  $6\pi \eta {a^2}v$
• B
  $\frac{6\eta v}{\pi a}$
• C
  $6\pi \eta av$
• D
  $\frac{\pi \eta v}{6 a^3}$