The magnetic induction at the centre O shown in the figure is

• A

  $\frac{{\mathrm{\mu }}_0\mathrm{I}}{2\mathrm{a}}+\frac{{\mathrm{\mu }}_0\mathrm{I}}{2\mathrm{b}}\otimes$

• B

  $\frac{3{\mathrm{\mu }}_0\mathrm{I}}{8\mathrm{a}}+\frac{{\mathrm{\mu }}_0\mathrm{I}}{8\mathrm{b}}\otimes$

• C

  $\frac{3{\mathrm{\mu }}_0\mathrm{I}}{8\mathrm{a}}-\frac{{\mathrm{\mu }}_0\mathrm{I}}{8\mathrm{b}}\otimes$

• D

  $\frac{{\mathrm{\mu }}_0\mathrm{I}}{2\mathrm{a}}-\frac{{\mathrm{\mu }}_0\mathrm{I}}{2\mathrm{b}}\otimes$