Capacitor

In the following circuits, it is given that $\mathrm{R}_1=1 \Omega, \mathrm{R}_2=2 \Omega, \mathrm{C}_1=2 \mu \mathrm{~F}$ and $\mathrm{C}_2=4 \mu \mathrm{~F}$.

The time constants (in $\mu \mathrm{s}$ ) for the circuits I, II, III are, rcspectively.