With a standard rectangular bar magnet of length (l), breath (b ; b < < l) and magnetic moment M, the time period of the magnet in a vibration magnetometer is 4 s. If the magnet is cut normal to its length into four equal pieces, the time period (in seconds) with one of the pieces is :

Question

$$\begin {array} {1 1} (a)\;16 & \quad (b)\;2 \\ (c)\;1 & \quad (d)\;1/4 \end {array}$$