18. A charged capacitor connected to a resistor and a switch, which is open for t＜0. After the switch is closed at t=0, the capacitor C is discharged through the resistor R. The energy stored in the capacitor decreases with time as it discharges. After how many time constants is this stored energy one eighth of its initial value? (ln2=0.693) (A) 0.347 RC (B) 0.693 RC (C) 1.040 RC (D) 1.386 RC (E) 2.079 RC