An audio frequency of $4\times 10^3$Hz is used to modulate a carrier wave of frequency 1MHz.The band width is

Question

$\begin{array}{1 1}(a)\;4\times 10^3Hz\\(b)\; 4\times 10^6Hz\\(c)\;8\times 10^3Hz\\(d)\;8\times 10^6Hz\end{array}$