A bar magnet is cut into two equal halves by a plane parallel to the magnetic axis. Of the following physical quantities, the one which remains unchanged is

Two points $A$ and $B$ are situated perpendicular to the axis of $4 \mathrm{cm}$ long bar magnet at distances $\mathrm{x}$ and $3\mathrm{x}$ from its centre on opposite sides. The ratio of magnetic inductions at $A$ and $B$ will be equal to

A bar magnet of magnetic moment $M$ is placed at a distance $D$ with its axis along positive $X$-axis. Likewise, second bar magnet of magnetic moment $M$ is placed at a distance $2D$ on positive $Y$-axis and perpendicular to it as shown in the figure. The magnitude of magnetic field at the origin is $\left|\overrightarrow{B}\right|=\alpha \left[\frac{{\mu }_0}{4\pi }\frac{M}{D^3}\right]$. The value of $\alpha$ must be (Assume $D>>$ where $l$ is the length of magnets)