At B the voltage now increases until the voltage on the anode of the diode reduces and is no longer positive with respect to the voltage on its cathode thereby disconnecting the capacitor from the input voltage.
As the capacitor cannot discharge back into the supply via the diode its only discharge path is into the load.
By making the value of the capacitor large enough, the rate at which the voltage falls will be low.
The capacitor will continue to discharge until the anode of the diode is positive with respect to its cathode.
The diode then being forward biased recharges the capacitor to a maximum value D.
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