Dielectrics: A parallel$-$plate capacitor has a capacitance of $10$ mF and is charged with a $20-$V power supply. The power supply is then removed and a dielectric material of dielectric constant $4\mathrm{.}0$ is used to fill the space between the plates. What is the voltage now across the capacitor?

$80$ V

$05\mathrm{.}0$ v

$02\mathrm{.}5$ V

$10$ V

$020$v