Two cells of emf’s approximately 5V and 10V are to be accurately

Two cells of emf’s approximately 5V and 10V are to be accurately compared using a potentiometer of length 400 cm.

(a) the battery that runs the potentiometer should have voltage of 8V

(b) the battery of potentiometer can have a voltage of 15V and R adjusted so that the potential drop across the wire slightly exceeds 10V

(c) the first portion of 50 cm of wire itself should have a potential drop of 10V

(d) potentiometer is usually used for comparing resistances and not voltages