(i) The pairs (1) and (3) will give rise to beats as they represent two simple harmonic waves of slightly different frequency, travelling in the same direction.
(ii) The pairs (1) and (2) will. produce distinctive interference as they represent two identical waves with a phase difference of n, travelling in the same direction.
(iii) The pairs (3) and (4) represent identical waves travelling in opposite directions. They will give rise to stationary waves