A cylinderical tube, open at one end and closed at the other, is in acoustic unison (resonance) with an external source of sound of singal frequency held at the open end of the tube, in its fundamental note.
Then :

Options

A. the displacement wave from the source gets reflected with a phase change of `pi` at the closed end
B. the pressure wave from the source get reflected without a phase change at the closed end
C. the wave reflected from the closed end again gets reflected at the open end
D. All the above

Answer

D