A fixed-resistance electrical device of $20\,\Omega$ is joined in series with a variable resistor and a battery with e.m.f. $16\,\text{V}$ and negligible internal resistance. The electrical device dissipates $4.0\,\text{W}$ of power. What resistance does the variable resistor have?
- A$16\,\Omega$
- B$36\,\Omega$
- C$44\,\Omega$
- D$64\,\Omega$