When a rubber band is stretched by a distance $x$, it exerts a restoring force of magnitude $F=a x+$ $b x^{2}$, where $a$ and $b$ are constants. The work done in stretching the unstretched rubber band by $L$ is:
[2014]
(A) $a L^{2}+b L^{3}$
(B) $\frac{1}{2}\left(a L^{2}+b L^{3}\right)$
(C) $\frac{a L^{2}}{2}+\frac{b L^{3}}{3}$
(D) $\frac{1}{2}\left(\frac{2 L^{2}}{2}+\frac{b L^{2}}{3}\right)$