Two batteries of different $e.m.f.'s$ and internal resistance connected in series with each other and with an external load resistor. The current is $3.0 \,A$. When the polarity of one battery is reversed, the current becomes $1.0 \,A$. The ratio of the $e.m.f.'s$ of the two batteries is ............
  • A$2.5$
  • B$2$
  • C$1.5$
  • D$1$
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