Which of the following statements is true?
(a) The capacitance of a capacitor is the total amount of charge it can store.
(b) The equivalent capacitance of two capacitors in series is the sum of the two capacitances.
(c) A dielectric material inserted between the conductors of a capacitor increases its capacitance.
(d) The electrostatic energy stored by a capacitor is equal to the ratio of the charge on either conductor to the potential difference between the conductors.