A bar magnet of moment M gives a time period 'T' at a place in a vibration magnetometer. Four such similar bar magnets are placed in the frame one over the other out of which one magnet is placed with opposite polarity. The new time period is :

Question

$\begin {array} {1 1} (1)\;\large\frac{T}{\sqrt 2} & \quad (2)\;\sqrt 2T \\ (3)\;T & \quad (4)\;2T \end {array}$