A cell has internal resistance $r$ and electromotive force (e.m.f.) $E$. The cell is placed in series with an ammeter and a variable resistor with resistance $R$. When $R$ is $10\,\Omega$ the ammeter shows $0.3\,\text{A}$. When $R$ is $5\,\Omega$ the ammeter shows $0.4\,\text{A}$. What is the value of $E$?
- A$0.5\,\text{V}$
- B$2\,\text{V}$
- C$3\,\text{V}$
- D$6\,\text{V}$