The dipole moment of a circular loop carrying a current I, is m and the magnetic field at the centre of the loop is $B_t$. When the dipole moment is double by keeping the current constant, the magnetic field at the centre of loop is $B_z$. The ratio $\frac{B_1}{B_2}$ is:

Question

( A ) $\sqrt{2}$
( B ) $\sqrt{3}$
( C ) $2$
( D ) $\frac{1}{\sqrt{2}}$