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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 ρ and σ respectively, and the viscosity of the liquid is η).

(a) r2g9ηρ-2σ                     (b) r2g9η2ρ-σ

(c) r2g9ηρ-σ                        (d) 2r2g9ηρ-σ