Two concentric circular coils `A` and `B` have radii `25cm` and `15cm` and carry currents `10A` and `15A` respectively. A has 24 turns and `B` has 18 turns. The direction of currents are in opposite order. The magnetic induction at the common centre of the coil is
A. `120 mu_(0)T`
B. `480 mu_(0)T`
C. `420mu_(0)T`
D. `mu_(0)`