A thin rod of length $'L'$ is lying along the $x$-axis with its ends at $x=0$ and $x=L$. Its linear density (mass/length) varies with $x$ as $k{\left( {{x \over L}} \right)^n},$ where $n$ can be zero or any positive number. If the position ${X_{CM}}$ of the center of mass of the rod is plotted against $'n',$ which of the following graphs best approximates the dependence of ${X_{CM}}$ on $n$?