A part of a long wire carrying a current i is bent into a circle of radius r as shown in figure. The net magnetic field at the centre O of the circular loop is [UPSEAT 2002]

• A.            \[\frac{{{\mu }_{o}}i}{4r}\]
• B.            \[\frac{{{\mu }_{o}}i}{2r}\]
• C.            \[\frac{{{\mu }_{o}}i}{2\pi r}(\pi +1)\]
• D.            \[\frac{{{\mu }_{o}}i}{2\pi r}(\pi -1)\]