A potential divider circuit is intended to sense the temperature difference between two separate locations. The cell has an electromotive force (e.m.f.) of $20\,\text{mV}$ and negligible internal resistance. At the start, thermistors $X$ and $Y$ are at the same temperature and therefore have equal resistance. The voltmeter shows $10\,\text{mV}$. $X$ is then moved into a cold environment, so its resistance becomes twice its original value. $Y$ is then moved into a warm environment, so its resistance falls to half its original value. What is the voltmeter reading now?
- A$4\,\text{mV}$
- B$5\,\text{mV}$
- C$15\,\text{mV}$
- D$16\,\text{mV}$