A Spherical Raindrop Evaporates at a Rate Proportional to Its $\frac{d V}{d t}$

Multiple Choice

A spherical raindrop evaporates at a rate proportional to its surface area.If V = volume of the raindrop and S = surface area, which of the following is a differential equation for $\frac{d V}{d t}$ ?

A) $\frac{d V}{d t}=k S$ with k a negative constant
B) $\frac{d V}{d t}=\frac{k}{S}$ with k a negative constant
C) $\frac{d V}{d t}=k S$ with k a positive constant
D) $\frac{d V}{d t}=\frac{k}{S}$ with k a positive constant