For a raindrop of mass $m$ descending with speed $v$, the resultant force $F$ is described by the equation $F = mg - kv^2$, where $k$ is a constant and $g$ is the acceleration of free fall. What is the raindrop’s velocity when it has reached a constant (terminal) velocity?
- A$\sqrt{\frac{k}{mg}}$
- B$\frac{k}{mg}$
- C$\sqrt{\frac{mg}{k}}$
- D$\frac{mg}{k}$