A spherical raindrop evaporates at a rate proportional to its surface area. Write a differential equation for the volume of the raindrop as a function of time.
a) dV/dt = -kV(2/3)
b) dV/dt = -kV(3/2)
c) dV/dt = -kV(1/2)
d) dV/dt = -kV(2/5)