Two north poles each of pole strength m and a south pole of pole strength m are placed at the three corners equilateral triangle of side a. The intensity of magnetic induction field strength at the centre of the triangle is

Options

A. zero
B. `(mu_0 m^2)/( 4 pi d^2)`
C. `[(2 sqrt(2) +1)/( 2) ] ( mu_0)/( 4 pi ) (m^2) /( d^2)`
D. `[(2 sqrt(2) -1)/(2) ] (mu_0)/( 4pi ) (m^2)/( d^2)`