A resistor $R$ is joined in series with two cells, each of electromotive force (e.m.f.) $E$ but with different internal resistances $r_1$ and $r_2$. The voltmeter reading is $0\,\text{V}$. What is the resistance of $R$?
- A$0$
- B$r_1 - r_2$
- C$r_1 + r_2$
- D$\dfrac{r_1 r_2}{r_1 + r_2}$