The direction of magnetic lines of force close to a straight conductor carrying current will be

The magnetic induction at point $O$ for the current-carrying wire shown in the figure is:

Shown in the figure is a conductor carrying a current $I$. The magnetic field intensity at the point $O$ (common centre of all the three arcs) is

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

The magnetic field at the centre of a circular coil of radius $R$,carrying current $2 \ A$ is $B_1$. The magnetic field at the centre of another coil of radius $3R$ carrying current $4 \ A$ is $B_2$. The ratio $B_1: B_2$ is

If we double the radius of a coil keeping the current through it unchanged,then the magnetic field at any point at a large distance from the centre becomes approximately