When a rubber-band is stretched by a distance\[x\], it exerts a restoring force of magnitude \[F=ax+b{{x}^{2}}\], where a and b are constants. The work done in stretching the unstretched rubber band by \[L\] is,

• A. \[\frac{a{{L}^{2}}}{2}+\frac{b{{L}^{3}}}{3}\]       
• B. \[\frac{1}{2}\left( \frac{a{{L}^{2}}}{2}+\frac{b{{L}^{3}}}{3} \right)\]
• C. \[a{{L}^{2}}+b{{L}^{3}}\]             
• D. \[\frac{1}{2}(a{{L}^{2}}+a{{L}^{3}})\]