# A drop of liquid evaporates at a rate proportional to its area of surface. If the radius initially is $4\;mm$ and $5$ minutes later, the radius is reduced to $2\; mm$, find radius of drop as a function of time ?
$(a)\;R=\frac{20}{t+5} \\ (b)\;R= \frac{t+5}{20} \\ (c)\;R=\frac{10}{t+5} \\ (d)\;R=\frac{10}{t+4}$