The magnetic induction at the centre of a current-carrying circular coil of radius $10 \mathrm{cm}$ is $5\sqrt{5}$ times the magnetic induction at a point on its axis. The distance of the point from the centre of the coil in $\mathrm{cm}$ is

Magnetic field at point $O$ due to the given structure is

Magnetic field at the centre $O$ due to the given structure is

Two thick wires and two thin wires, all of the same material and same length, form a square in three different ways $A, B$ and $C$ as shown in the figure. With correct connections shown, the magnetic field at the centre of the square is zero in case of

Which of the following materials obeys the nature of given voltage-current $(V-I)$ graph under constant temperature?