[单选题]
A ball of mass $m$ is falling vertically through the air. The drag force acting on the ball is given by $kv^2$ where $k$ is constant and $v$ is the speed of the ball. What is the maximum speed reached by the ball?
A. $\frac{k}{mg}$
B. $\frac{mg}{k}$
C. $\sqrt{\frac{k}{mg}}$
D. $\sqrt{\frac{mg}{k}}$
