Two cells of emf $E_{1}$ and $E_{2}$ are joined in opposition (such that $E_{1} > E_{2}$). If $r_{1}$ and $r_{2}$ are the internal resistances and $R$ is the external resistance,then the terminal potential difference across the external resistance $R$ is:

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