A circular coil carrying a certain current produces a magnetic field $B_0$ at its centre. The coil is now rewound so as to have $3$ turns and the same current is passed through it. The new magnetic field at the centre 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?