$A$ conducting wire carrying current $I$ is arranged as shown in the figure. Find the magnetic field at point $O$.

• A
  $\frac{\mu_{0} I}{12}\left[\frac{1}{R_{1}}-\frac{1}{R_{2}}\right]$
• B
  $\frac{\mu_{0} I}{12}\left[\frac{1}{R_{1}}+\frac{1}{R_{2}}\right]$
• C
  $\frac{\mu_{0} I}{6}\left[\frac{1}{R_{1}}-\frac{1}{R_{2}}\right]$
• D
  $\frac{\mu_{0} I}{6}\left[\frac{1}{R_{1}}+\frac{1}{R_{2}}\right]$