The ratio of the magnetic field at the centre of a current carrying coil to its magnetic moment is $ x$. If the current and the radius are both doubled, the new ratio will become

Question

$\begin {array} {1 1} (a)\;8 \: x & \quad (b)\;4\: x \\ (c)\;\large\frac{x}{8} & \quad (d)\;\large\frac{x}{4} \end {array}$