In a large room, a person receives direct sound waves from a source 120 metres away from him. He also receives waves from the same source which reach him, being reflected from the 25 metre high ceiling at a point halfway between them. The two waves interfere constructively for wavelength of [Roorkee 1982]

• A.            20, 20/3, 20/5 etc                
• B.            10, 5, 2.5 etc
• C.            10, 20, 30 etc                        
• D.            15, 25, 35 etc