13. A dielectric material is inserted between the plates of a charged capacitor. Which of the followings statements about the effects of the dielectric is TRUE?
a) If the capacitor is not connected to a battery, the magnitude of the electric field between the plates increases.
b) If the capacitor is connected to a battery, the charge on the plates remains constant.
c) If the capacitor is not connected to a battery, the voltage between the plates remains constant.
d) If the capacitor is not connected to a battery, the charge on the plates remains constant.