A student performs an experiment for determination of $g\left(=\frac{4 \pi^2 l}{\mathrm{~T}^2}\right), l=1 m$, and he commits an error of $\Delta l$. For T , he takes the time of $n$ oscillations with the stop watch of least count $\Delta \mathrm{T}$ and he commits a human error of 0.1 s . For which of the following data, the measurement of $g$ will be most accurate?

Match the following columns.

	Column I		Column II
(A)	Dielectric ring uniformly charged.	(P)	Time independent electrostatic field out of system.
(B)	Dielectric ring uniformly charged rotating with angular velocity $$\omega$$.	(Q)	Magnetic field.
(C)	Constant current in ring $$io$$	(R)	Induced electric field.
(D)	$$i=i_0\cos\omega t$$	(S)	Magnetic moment.

A solid sphere of radius $R$ has moment of inertia $I$ about its geometrical axis. If it is melted into a disc of radius $r$ and thickness $t$. If its moment of inertia about the tangential axis (which is perpendicular to plane of the disc), is also equal to $I$, then the value of $r$ is equal to

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.