The resistances of three parts of a circular loop are as shown in figure. The magnetic field at the center ‘O’ is (current enters at A and leaves at B and C as shown)

• A

  $\frac{{\mu }_{0}i}{6a}$

• B

  $\frac{{\mu }_{0}i}{3a}$

• C

  $\frac{2}{3}\frac{{\mu }_{0}i}{a}$

• D

  $zero$