A potentiometer wire of length `100 cm` having a resistance of `10 Omega` is connected in series with a resistance `R` and a cell of emf `2V` of negligible internal resistance. A source of emf
of `10 mV` is balanced against a length of `40 cm` of the potentiometer wire. What is the value of resistance `R` ?
A. `760 Omega`
B. `640 Omega`
C. `790 Omega`
D. `840 Omega`