Two sources of equal $emf$ $(\varepsilon)$ are connected in series to 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 having internal resistance $R_2$ is zero, then:

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

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