Magnetic field at a distance $r$ from an infinitely long straight conductor carrying a steady current varies as

In the previous question,if the current is $i$ and the magnetic field at $D$ has magnitude $B$,

$A$ long straight wire carries a current of $35\; A$. What is the magnitude of the field $B$ at a point $20\; cm$ from the wire?

If the current flowing through the shell of the previous objective is equal to $i$,then the energy density at a point at a distance $2R$ from the axis of the shell varies according to which graph?

Two concentric circular coils of $10$ turns each are situated in the same plane. Their radii are $20 \, cm$ and $40 \, cm$ and they carry respectively $0.2 \, A$ and $0.3 \, A$ current in opposite directions. The magnetic field at the centre is $(\mu_0 = 4 \pi \times 10^{-7} \, T \cdot m/A)$

In the following figure,a wire bent in the form of a regular polygon of $n$ sides is inscribed in a circle of radius $a$. The net magnetic field at the centre will be