A man is standing not exactly at the centre, in between two parallel walls separated by a distance 990 m. He fires a gun and hears the first echo after 2 s, the time taken to hear 3rd echo after firing is __________ s, (take velocity of sound in air 330 ms–1)
(a) 5                   (b) 6                (c) 7                    (d) 8

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