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 shows three cases: in all cases,the circular part has radius $r$ and straight ones are infinitely long. For the same current,the $B$ field at the centre $P$ in cases $1$,$2$,and $3$ have the ratio:

• 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
  $\left( -\frac{\pi}{2} - 1 \right) : \left( \frac{\pi}{2} - \frac{1}{4} \right) : \left( \frac{3\pi}{4} + \frac{1}{2} \right)$