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$\frac{{\pi \eta v}}{{6 a^3}}$
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