An empty capacitor $)=(1\mathrm{.}0$ is connected to a $6$ V battery and charged up$.$ The capacitor is then disconnected from the battery and a slab of dielectric material $)=(3\mathrm{.}0$ is inserted between the plates. Find the new potential difference across the plates. $($Hint: the capacitor will maintain the same charge when disconnected from the battery!$).$

A$)18$ V

B$)6$ V

C$)3$ V

D$)2$ V