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 h). [Kerala PET 2005]

• A.            \[\frac{{{r}^{2}}g}{9\eta }(\rho -2\sigma )\]               
• B.            \[\frac{{{r}^{2}}g}{9\eta }(2\rho -\sigma )\]
• C.            \[\frac{{{r}^{2}}g}{9\eta }(\rho -\sigma )\]                 
• D.            \[\frac{2{{r}^{2}}g}{9\eta }(\rho -\sigma )\]