According to the Biot-Savart law,what is the magnetic field at a point on the axis of a current-carrying wire?

The figure shows a square loop $ABCD$ with edge length $a$. The resistance of the wire $ABC$ is $r$ and that of $ADC$ is $2r$. The value of the magnetic field at the centre of the loop is:

$A$ wire $A$, bent in the shape of an arc of a circle, carrying a current of $2 \, A$ and having radius $2 \, cm$, and another wire $B$, also bent in the shape of an arc of a circle, carrying a current of $3 \, A$ and having radius $4 \, cm$, are placed as shown in the figure. The ratio of the magnetic fields due to the wires $A$ and $B$ at the common centre $O$ is

$A$ current passing through a circular coil of two turns produces a magnetic field $B$ at its centre. The coil is then rewound so as to have four turns and the same current is passed through it. The magnetic field at its centre now is

$A$ thin charged rod is bent into the shape of a small circle of radius $R$,the charge per unit length of the rod being $\lambda$. The circle is rotated about its axis with a time period $T$,and it is found that the magnetic field at a distance $d$ away $(d >> R)$ from the center and on the axis varies as $\frac{R^{m}}{d^{n}}$. The values of $m$ and $n$ respectively are:

Two straight horizontal parallel wires are carrying the same current in the same direction, and $d$ is the distance between the wires. You are provided with a small freely suspended magnetic needle. At which of the following positions will the orientation of the needle be independent of the magnitude of the current in the wires?