$A$ parallel plate capacitor is made of two plates of length $l$,width $w$ and separated by distance $d$. $A$ dielectric slab (dielectric constant $K$) that fits exactly between the plates is held near the edge of the plates. It is pulled into the capacitor by a force $F = -\frac{\partial U}{\partial x}$ where $U$ is the energy of the capacitor when the dielectric is inside the capacitor up to distance $x$ (See figure). If the charge on the capacitor is $Q$,then the force on the dielectric when it is near the edge is

• A
  $\frac{{{Q^2}d}}{{2w{l^2}{\varepsilon _0}}}K$
• B
  $\frac{{{Q^2}w}}{{2d{l^2}{\varepsilon _0}}}\left( {K - 1} \right)$
• C
  $\frac{{{Q^2}d}}{{2w{l^2}{\varepsilon _0}}}\left( {K - 1} \right)$
• D
  $\frac{{{Q^2}w}}{{2d{l^2}{\varepsilon _0}}}K$