$A$ parallel plate capacitor has plates of length $l$,width $w$,and separation $d$. It is connected to a battery of emf $V$. $A$ dielectric slab of the same thickness $d$ and dielectric constant $k = 4$ is being inserted between the plates. At what length $x$ of the slab inside the plates will the energy stored in the capacitor be two times the initial energy stored?

• A
  $\frac{l}{4}$
• B
  $\frac{l}{2}$
• C
  $\frac{l}{3}$
• D
  $\frac{2l}{3}$