Two sources of equal emfs $E$ are connected in series. This combination is connected to an external resistance $R$. The internal resistances of the two sources are $r_{1}$ and $r_{2}$ $(r_{1} > r_{2})$. If the potential difference across the source of internal resistance $r_{1}$ is zero,then the value of $R$ will be:

• A
  $r_{1} - r_{2}$
• B
  $\frac{r_{1} r_{2}}{r_{1} + r_{2}}$
• C
  $\frac{r_{1} + r_{2}}{2}$
• D
  $r_{2} - r_{1}$