Two sources of equal $emf$ $E$ are connected in series with an external resistance $R$. The internal resistances of the two sources are $R_1$ and $R_2$ $(R_2 > R_1)$. If the potential difference across the source of internal resistance $R_2$ is zero,then the value of $R$ is:

  • A
    $R = \frac{R_1 R_2}{R_1 + R_2}$
  • B
    $R = \frac{R_1 R_2}{R_1 - R_2}$
  • C
    $R = R_2 \frac{(R_1 + R_2)}{(R_1 - R_2)}$
  • D
    $R = R_2 - R_1$

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