A non conducting ring (of mass $m,$ radius $r,$ having charge $Q$) is placed on a rough horizontal surface (in a region with transverse magnetic field). The field is increasing with time at the rate $R$ and coefficient of friction between the surface and the ring is $\mu .$ For ring to remain in equilibrium $\mu$ should be greater than
  • A$\frac{{QrR}}{{mg}}$
  • B$\frac{{QrR}}{{2mg}}$
  • C$\frac{{QrR}}{{3mg}}$
  • D$\frac{{2QrR}}{{mg}}$
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