A potentiometer is used for the comparison of $e.m.f.$ of two cells ${E_1}$ and ${E_2}$. For cell ${E_1}$ the no deflection point is obtained at $20\,cm$ and for ${E_2}$ the no deflection point is obtained at $30\,cm$. The ratio of their $e.m.f.$'s will be
  • A$2/3$
  • B$1/2$
  • C$1$
  • D$2$
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