Spherical balls of radius 'r' are falling in a viscous fluid of viscosity '$\mathrm{\eta }$' with a velocity 'v'. The retarding viscous force acting on the spherical ball is

• A

  Inversely proportional to 'r' but directly proportional to velocity 'v'

• B

  Directly proportional to both radius 'r' and velocity 'v'

• C

  Inversely proportional to both radius 'r' and velocity 'v'

• D

  Directly proportional to 'r' but inversely proportional to 'v'