In the figure, what is the magnetic field at the point O?

• A

  $\frac{{\mu }_{0}I}{4\pi r}$

• B

  $\frac{{\mathrm{\mu }}_0\mathrm{I}}{4\mathrm{\pi r}}+\frac{{\mathrm{\mu }}_0\mathrm{I}}{2\mathrm{\pi r}}$

• C

  $\frac{{\mathrm{\mu }}_0\mathrm{I}}{4r}+\frac{{\mathrm{\mu }}_0\mathrm{I}}{4\mathrm{\pi r}}$

• D

  $\frac{{\mathrm{\mu }}_0\mathrm{I}}{4r}-\frac{{\mathrm{\mu }}_0\mathrm{I}}{4\mathrm{\pi r}}$