The equation of a stationary wave is given by
$y = 6 \sin \left(\frac{\mathrm{\pi x}}{3}\right)\cos 40\mathrm{\pi t}$
where y and x are given in cm and time t in second, the frequency of the component progressive wave is:

• A

  20 Hz

• B

  40 Hz

• C

  ($\frac{1}{20}$) Hz

• D

  $\left(\frac{1}{10}\right) \mathrm{Hz}$