A wire is bent in the form of a quadrant of circle of radius 'a' as shown in the Fig. 8.93. The magnetic field at centre O due to current i is :

• A

  $\frac{{\mathrm{\mu }}_0}{4\mathrm{\pi }}.\frac{2\mathrm{\pi i}}{\mathrm{a}}$

• B

  $\frac{{\mathrm{\mu }}_0}{4\mathrm{\pi }}.\frac{\mathrm{\pi i}}{\mathrm{a}}$

• C

  $\frac{{\mathrm{\mu }}_0}{4\mathrm{\pi }}.\frac{\mathrm{\pi i}}{2\mathrm{a}}$

• D

  zero