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 time period of the component progressive wave is:

• A

  20 s

• B

  40 s

• C

  $\left(\frac{1}{20}\right)\mathrm{s}$

• D

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