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The parallel-plate capacitor shown in the figure has movable plates. The capacitor is charged so that the energy stored in it is $E$ when the plate separation is $d$. The capacitor is then isolated electrically and the plate are moved such that the plate separation becomes $2d$.

At this new plate separation, what is the energy stored in the capacitor, neglecting fringing effects?

1. $2E$
2. $\sqrt{2}E$
3. $E$
4. $E/2$