a circuit contains a d-cell battery, a switch, a 20-ω resistor, and three 20-mf capacitors. the capacitors are connected in parallel, and the parallel connection of capacitors are connected in series with the switch, the resistor and the battery. (a) what is the equivalent capacitance of the circuit? (b) what is the rc time constant? (c) how long before the current decreases to 50% of the initial value once the switch is closed?