Stokes' law states that viscous force depends upon radius r, coefficient of viscosity η, and terminal velocity v. If constant k = 6π, use dimensions to obtain the relation:

a) [F] = [η] × [r] × [v]
b) [F] = [η] × [r²] × [v]
c) [F] = [η] × [r] × [v²]
d) [F] = [η] × [r²] × [v²]