A potentiometer together with a driver cell of electromotive force (e.m.f.) $E$ is used to determine the e.m.f. of a new cell. The sliding contact at $P$ is adjusted along the resistance wire $QR$ until the galvanometer shows zero deflection. What condition must be satisfied for the e.m.f. of the new cell to be measured with accuracy?
- AThe e.m.f. of the driver cell must be less than the e.m.f. of the new cell.
- BThe galvanometer must have a large resistance.
- CThe internal resistance of the new cell must be zero.
- DThe resistance per unit length of the wire $QR$ must be constant.