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The ratio of the magnetic field at the centre of a current carrying coil of the radius a and at a distance ‘a’ from centre of the coil and perpendicular to the axis of coil is

(a) $\frac{1}{\sqrt{2}}$                              (b) $\sqrt{2}$

(c) $\frac{1}{2\sqrt{2}}$                             (d) $2\sqrt{2}$