An electrical device with a fixed resistance of $20\,\Omega$ is placed in series with a variable resistor and a battery whose electromotive force (e.m.f.) is $16\,\text{V}$ and whose internal resistance is negligible. Determine the resistance of the variable resistor when the power dissipated by the electrical device is $4.0\,\text{W}$?
- A$16\,\Omega$
- B$36\,\Omega$
- C$44\,\Omega$
- D$60\,\Omega$