Mark the CORRECT statement(s):-

When a high pressure compression pulse reaches water surface in a resonance column, it reflects as a low pressure rare fraction pulse due to a phase change of  $\pi$.

When two waves of different frequency  $f_1$ and  $f_2$ are superimposed at a point, the variation of intensity takes place with frequency  $f_1-f_2$.

When a tuning fork is struck by a hammer a transverse wave sets in the prong while a longitudinal wave sets in the stem (handle) of tuning fork.

When an observer is moving, relative speed of the sound waves with respect to the observer is changed, while due to motion of source, the sound wavelength of the wave changes.