What is the ratio of the magnetic field at point $O$ in the given figures?

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