GATE MT 2019 Q26

Terminal rise velocity of a spherical shaped solid in a liquid obeys the following functional relationship: U=f(d,W,μ,ρ) where μ is the terminal rise velocity, d is the diameter of the solid, W is the apparent weight of the solid, μ is the viscosity of liquid and ρ is the density of liquid. According to Buckingham Π theorem, the number of independent dimensionless variables needed to describe the phenomenon is_________.