A potentiometer circuit is employed to determine the electromotive force (e.m.f.) of a cell $X$. The e.m.f. of cell $X$ is approximately $0.50\,\text{V}$. The driver cell has an e.m.f. of $2.0\,\text{V}$ and negligible internal resistance. The sliding contact is then shifted along the uniform resistance wire from end $Q$ to end $R$ until it reaches a position $P$ where the galvanometer shows zero. What expression gives the approximate length $QP$?
- A$\frac{QR}{4}$
- B$\frac{QR}{3}$
- C$\frac{2QR}{3}$
- D$\frac{3QR}{4}$