In a circuit with two batteries, both of negligible internal resistance, and two resistors arranged as shown, the galvanometer shows a current reading of zero. What is the electromotive force (e.m.f.) of battery $X$?
- A$6.0\,\text{V}$
- B$8.0\,\text{V}$
- C$16.0\,\text{V}$
- D$18.0\,\text{V}$