[" 25.Two concentric circular coils "A" and "B" have radis "],[25cm" and "15cm" and carry currents "10A" and "15c" of "],[" 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 couls "],[" is "],[[" 1) "120 mu_(0)T," 2) "480 mu_(0)T],[" 3) "80 mu_(0)T," 4) "420 mu_(0)T]]