Two sources of equal $emf$ are connected 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
    $R = R_1 R_2 / (R_1 + R_2)$
  • B
    $R = R_1 R_2 / (R_2 - R_1)$
  • C
    $R = R_2 \times (R_1 + R_2) / (R_2 - R_1)$
  • D
    $R = R_2 - R_1$

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