A solid ball of density `rho_(1)` and radius r falls vertically through a liquid of density `rho_(2)`. Assume that the viscous force acting on the ball is `F = krv`, where `k` is a constant and v its velocity. What is the terminal velocity of the ball ?
A. `(4 pi g r^(2) (rho_(1) - rho_(2))/(3k))`
B. `(2 pi r (rho_(1) - rho_(2))/(2gk))`
C. `(2 pi g (rho_(1) + rho_(2))/(3gr^(2)k))`
D. None of these
