Spherical balls of radius R are falling in a viscous fluid of viscosity Tl with a velocity v. The retarding viscous force acting on the spherical bali is: 

• A

  directly proportional to R but inversely 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

  inversely proportional to R but directly proportional to velocity v