The center of mass of a thin rectangular plate (fig - $x$) with sides of length $a$ and $b$, whose mass per unit area ($\sigma$) varies as $\sigma = \frac{\sigma_0 x}{ab}$ (where $\sigma_0$ is a constant), would be __________.
$\left( \frac{1}{3}a, \frac{b}{2} \right)$
$\left( \frac{2}{3}a, \frac{b}{2} \right)$
$\left( \frac{a}{2}, \frac{b}{2} \right)$
$\left( \frac{2}{3}a, \frac{2}{3}b \right)$