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 amplitude of progressive wave is:

• A

  6 cm

• B

  3 cm

• C

  12 cm

• D

  2 cm