The diagram depicts a variable resistor $R$ together with two fixed resistors joined in series in a circuit so that they function as a potential divider. The cell with electromotive force (e.m.f.) $6.0\text{ V}$ has negligible internal resistance. A cell with e.m.f. $2.0\text{ V}$ and a galvanometer are linked into the potential divider. As the resistance of $R$ is changed, the galvanometer shows zero. What is the resistance of resistor $R$?
- A$3.0\,\Omega$
- B$5.0\,\Omega$
- C$8.0\,\Omega$
- D$18\,\Omega$