Which of the following gives the value of the magnetic field according to Biot-Savart's law?

$A$ circular loop of radius $r$ of conducting wire connected with a voltage source of zero internal resistance produces a magnetic field $B$ at its centre. If instead, a circular loop of radius $2r$, made of the same material and having the same cross-section, is connected to the same voltage source, what will be the magnetic field at its centre?

$A$ circular loop of radius $R$ is carrying current $I$. The ratio of the magnetic field at the center of the circular loop to the magnetic field at a distance $R$ from the center of the loop on its axis is:

Two identical long current-carrying wires are bent into the shapes shown in the figures. If the magnitudes of the magnetic fields at the centers $P$ and $Q$ of the quarter-circular arcs are $B_1$ and $B_2$ respectively, then the ratio $\frac{B_1}{B_2}$ is:

Charge $q$ is uniformly spread on a thin ring of radius $R.$ The ring rotates about its axis with a uniform frequency $f \ Hz.$ The magnitude of magnetic induction at the center of the ring is

Two concentric rings are kept in the same plane. The number of turns in each ring is $25$. Their radii are $50 \text{ cm}$ and $200 \text{ cm}$,and they carry electric currents of $0.1 \text{ A}$ and $0.2 \text{ A}$ respectively in mutually opposite directions. The magnitude of the magnetic field produced at their centre is . . . . . . $\text{T}$.