$A$ small coil is introduced between the poles of an electromagnet so that its axis coincides with the magnetic field direction. The number of turns is $n$ and the cross-sectional area of the coil is $A$. When the coil turns through $180^o$ about its diameter,the charge flowing through the coil is $Q$. The total resistance of the circuit is $R$. What is the magnitude of the magnetic induction?
- A$\frac{QR}{nA}$
- B$\frac{2QR}{nA}$
- C$\frac{Qn}{2RA}$
- D$\frac{QR}{2nA}$
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