Three rings, each having equal radius R, are placed mutually perpendicular to each other and each having its centre at the origin of coordinate system. If current I is flowing through each ring, then the magnitude of the magnetic field at the common centre is
A. `sqrt3(mu_0I)/(2R)`
B. zero
C. `(sqrt2-1) (mu_0I)/(2R)`
D. `(sqrt3-sqrt2) (mu_0I)/(2R)`