An audio signal $20\sin(2\pi\times 400t)$ is used to modulating a carrier wave given by $40\sin(2\pi\times 40000t)$.The percentage modulation is

Question

$\begin{array}{1 1}(a)\;25\%\\(b)\; 40\%\\(c)\;10\%\\(d)\;50\%\end{array}$