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A straight wire carrying current i is turned into a circular loop. If the magnitude of magnetic moment associated with it in M.K.S. unit is M, the length of wire will be

(a) $4\mathrm{\pi iM}$                          (b) $\sqrt{\frac{4\mathrm{\pi M}}{i}}$

(c) $\sqrt{\frac{4\mathrm{\pi i}}{M}}$                        (d) $\frac{M\mathrm{\pi }}{4i}$