A raindrop with mass $m$ is moving straight down through the air at a constant speed $v$. The air exerts a retarding force $kv$ on the raindrop, where $k$ is a constant. The acceleration of free fall is $g$. Which expression gives the raindrop's kinetic energy?
- A$\frac{mg}{k}$
- B$\frac{mg^2}{2k^2}$
- C$\frac{m^3g^2}{k^2}$
- D$\frac{m^3g^2}{2k^2}$