The population of a country doubles in 40 years. Assuming that the rate of increase is proportional to number of inhabitants, find the number of years in which the population would treble itself?

Question

$\begin{array}{1 1}(A) \;20 \large\frac{\log 2}{\log 3}\; years \\(B) \;40 \large\frac{\log 2}{\log 3}\; years \\ (C) \;40 \large\frac{\log 3}{\log 2}\; years \\ (D) \;20 \large\frac{\log 3}{\log 2}\; years\end{array}$