A resistor R=200Ω is in series with a capacitor C1=8.0μF and a battery. A switch can connect an additional capacitor C2=4.0μF in parallel with C1. When the switch is open, the circuit contains only C1; when it is closed, both capacitors are present. By what factor does the RC time constant change when the switch is closed?
a) 1.5
b) 0.67
c) 0.33
d) 2.0