Magnetic Field Questions

A current loop $ABCD$ is held fixed on the plane of the paper. The arcs $BC$ (radius $=b$) and $DA$ (radius $=a$) of the loop are joined by two straight wires $AB$ and $CD$. A steady current $I$ is flowing in the loop. Angle made by $AB$ and $CD$ at the origin $O$ is $30^{\circ }$. The magnitude of the magnetic field ($B$) due to loop $ABCD$ at the origin ($O$) is: