The minimum distance of reflector surface from the source for listening the echo of sound is 

(1) 28 m

(2) 18 m

(3) 19 m

(4) 16.5 m

(4) 2d=v×t, where v = velocity of sound = 332 m/s

t = Persistence of hearing = 110sec.

d=v×t2=332×1102=16.5m

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