Which of the following graphs correctly represents the variation of magnetic field $B$ with distance $R$ from a long straight current-carrying conductor?

At what distance on the axis,from the centre of a circular current-carrying coil of radius $r$,does the magnetic field become $1/8$th of the magnetic field at the centre?

$A$ straight wire of length $\pi^2 \, m$ carries a current of $2 \, A$. The magnetic field due to it is measured at a point $1 \, cm$ away from it. If the wire is bent into a circle and carries the same current,what is the ratio of the magnetic field at its centre to the magnetic field measured in the first case?

Two long parallel wires carry currents $I_1$ and $I_2$ $(I_1 > I_2)$. When currents are flowing in the same direction,the magnetic field at a point midway between the wires is $6 \times 10^{-6} \ T$. If the direction of $I_2$ is reversed,the field at the midpoint becomes $3 \times 10^{-5} \ T$. The ratio $I_1 : I_2$ is

$PQRS$ is a square loop made of a uniform conducting wire. The current enters the loop at $P$ and leaves at $S$. Then the magnetic field will be

$A$ wire of resistance $R$ is bent in the form of a square of side $a$ as shown in the figure. Find the magnetic induction at the center of the square $O$ due to the current flowing through it.