A rectangular parallel-plate capacitor has a dielectric slab that partially fills the space between the plates as shown in fig. The energy of the capacitor with charge q is
 

• A

  $\frac{{\mathrm{q}}^2\mathrm{S}}{2{\mathrm{\epsilon }}_0\mathrm{b}}\times \frac{1}{[\mathrm{Ka}-(\mathrm{K}-1\left)\mathrm{x}\right]}$

• B

  $\frac{{\mathrm{q}}^2\mathrm{S}}{{\mathrm{\epsilon }}_0\mathrm{b}}\times \frac{1}{[\mathrm{Ka}-(\mathrm{K}-1\left)\mathrm{x}\right]}$

• C

  $\frac{{\mathrm{q}}^2\mathrm{S}}{2{\mathrm{\epsilon }}_0\mathrm{b}}[\mathrm{Ka}-(\mathrm{K}-1\left)\mathrm{x}\right]$

• D

  $\frac{{\mathrm{q}}^2\mathrm{S}}{2{\mathrm{\epsilon }}_0\mathrm{b}}\left[\frac{1}{\mathrm{Ka}}-\frac{1}{(\mathrm{K}-1)\mathrm{x}}\right]$