The magnetic field at the centre of a circular coil of radius r is \[\pi \] times that due to a long straight wire at a distance r from it, for equal currents. Figure here shows three cases : in all cases the circular part has radius r and straight ones are infinitely long. For same current the B field at the centre P in cases 1, 2, 3 have the ratio [CPMT 1989]

• A.            \[\left( -\frac{\pi }{2} \right):\left( \frac{\pi }{2} \right):\left( \frac{3\pi }{4}-\frac{1}{2} \right)\]
• B.            \[\left( -\frac{\pi }{2}+1 \right):\left( \frac{\pi }{2}+1 \right):\left( \frac{3\pi }{4}+\frac{1}{2} \right)\]
• C.            \[-\frac{\pi }{2}:\frac{\pi }{2}:3\frac{\pi }{4}\]
• D.            \[\left( -\frac{\pi }{2}-1 \right):\left( \frac{\pi }{2}-\frac{1}{4} \right):\left( \frac{3\pi }{4}+\frac{1}{2} \right)\]