Two straight long conductors AOB and COD are perpendicular to each other and carry currents I1 and I2. The magnitude of the mag. field at a point "P" at a distance "a" from the point "O" in a direction perpendicular to the plane ABCD is
(a) [(μ0)/(2πa)](I1 + I2)
(b) [(μ0)/(2πa)](I1 – I2)
(c) [(μ0)/(2 πa)](I12 + I22)1/2
(d) [(μ0)/(2 πa)][(I1I2)/(I1 – I2)]