What is the velocity v of a metallic ball of radius r falling in a tank of liquid at the instant when its acceleration is one-half that of a freely falling body ? (The densities of metal and of liquid are r and s respectively, and the viscosity of the liquid is `eta`).

Options

A. `(r^(2)g)/(9eta)(rho-2sigma)`
B. `(r^(2)g)/(9eta)(2rho-sigma)`
C. `(r^(2)g)/(9eta)(rho-sigma)`
D. `(2r^(2)g)/(9eta)(rho-sigma)`