An electric bulb rated for 500 watts at 100 volts is used in a circuit having a 200-volt supply. The resistance R that must be put in series with the bulb, so that the bulb draws 500 W is

• A.            \[10\,\Omega \]                 
• B.            \[20\,\Omega \]
• C.                                       Rated current through the circuit \[i=\frac{500}{100}=5A\]            Potential difference across R, \[100=5\times R\,\Rightarrow \,R=20\,\Omega \]
• D.            \[50\,\Omega \]