In the diagram shown below, what is the magnetic field at the point O? (straight wires have infinite length)

• A

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

• B

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

• C

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

• D

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