64. 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) [(r²g)/(9η)][ ρ -2σ]                         (b) [(r²g)/(9η)][ 2ρ -σ]

(c)  [(r²g)/(9η)][ ρ -σ]                          (d) [(2r²g)/(9η)][ ρ -σ]

Answer-c