3 points $O(0,0), P\left(a, a^{2}\right), Q\left(-b, b^{2}\right)(a>0, b>0)$ are on the parabola $y=x^{2} .$ Let $S_{1}$ be the area bounded by the line $P Q$ and the parabola and let $S_{2}$ be the area of the $\triangle O P Q$. the minimum value of $S_{1} / S_{2}$ is
(a) $4 / 3$
(b) $5 / 3$
(c) 2
(d) $7 / 3$