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, then the speed of the component progressive wave is:

• A

  20 cm/s

• B

  6 cm/s

• C

  120 cm/s

• D

  40 cm/s