Within a fire alarm system, thermistor T has a resistance of $2000\,\Omega$ at room temperature. As the temperature rises, its resistance falls. The alarm operates when the potential difference between X and Y becomes $4.5\text{ V}$.\n\nWhat is the resistance of the thermistor at the point when the alarm is triggered?
- A$90\,\Omega$
- B$150\,\Omega$
- C$250\,\Omega$
- D$1300\,\Omega$