A network is made by connecting four resistors with resistances $R$, $2R$, $3R$ and $4R$. As shown, a battery with negligible internal resistance and a voltmeter are joined to the resistor network. The voltmeter shows a reading of $2\,\text{V}$. What is the electromotive force (e.m.f.) of the battery?
- A$2\,\text{V}$
- B$4\,\text{V}$
- C$6\,\text{V}$
- D$10\,\text{V}$