A resistor with resistance $R$ is connected in a circuit with a variable resistor, a voltmeter and a power supply with electromotive force (e.m.f.) $E$. The resistance of the variable resistor rises from $0$ to $2R$. What change occurs in the voltmeter reading?
- AIt increases from $0$ to $\frac{1}{3}E$.
- BIt increases from $0$ to $\frac{1}{2}E$.
- CIt increases from $0$ to $\frac{2}{3}E$.
- DIt increases from $0$ to $E$.