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

   directly proportional to R  but inversely proportional to v

• B

   directly proportional to radius R and velocity v

• C

   inversely proportional to both radius R and
  velocity v

• D

   inversely proportional to radius r but directly proportional to velocity v