A $100\,\text{cm}$ potentiometer wire $QT$ is arranged in series with a $2.00\,\text{V}$ cell. A second circuit, made up of a $2.00\,\text{V}$ cell in series with resistors of resistance $4.00\,\Omega$ and $6.00\,\Omega$, is placed beside the potentiometer. The links $PQ$ and $RS$ are then connected so that the potential difference (p.d.) across the $4.00\,\Omega$ resistor is matched with the p.d. across a length $L$ of potentiometer wire. Both cells are assumed to have negligible internal resistance. What is the balance length $L$?
- A$0\,\text{cm}$
- B$40\,\text{cm}$
- C$60\,\text{cm}$
- D$100\,\text{cm}$