Two cells, with electromotive forces $E_1$ and $E_2$ and internal resistances $r_1$ and $r_2$, are connected to a resistor $R$ as shown. The terminal potential difference across cell 1 is zero. Which expression gives the resistance of resistor $R$?
- A$\frac{E_2 r_1 - E_1 r_2}{E_1}$
- B$\frac{E_2 r_1 - E_1 r_2}{E_2}$
- C$\frac{E_1 r_2 - E_2 r_1}{E_1}$
- D$\frac{E_1 r_2 - E_2 r_1}{E_2}$