A long copper tube of inner radius R carries a current i. The magnetic field B inside the tube is [MP PMT 1995]

• A.   \[\frac{{{\mu }_{0}}i}{2\pi R}\]     
• B.   \[\frac{{{\mu }_{0}}i}{4\pi R}\]
• C.   \[\frac{{{\mu }_{0}}i}{2R}\]   
• D.   Zero