The terminal velocity of a copper ball of radius $2.0 \; mm$ falling through a tank of oil at $20 \; ^{\circ}C$ is $6.5 \; cm \; s^{-1}$. Compute the viscosity of the oil at $20 \; ^{\circ}C$. Density of oil is $1.5 \times 10^{3} \; kg \; m^{-3}$,density of copper is $8.9 \times 10^{3} \; kg \; m^{-3}$.

• A
  $1.1 \times 10^{-1} \; kg \; m^{-1} \; s^{-1}$
• B
  $9.9 \times 10^{-1} \; kg \; m^{-1} \; s^{-1}$
• C
  $6.37 \times 10^{-2} \; kg \; m^{-1} \; s^{-1}$
• D
  $5.98 \times 10^{-1} \; kg \; m^{-1} \; s^{-1}$