The correct option is C 20 s
The distance between a rarefaction and the next compression is 10 cm i.e λ2=10 cm
Hence,
λ=20 cm = 0.2 m
The time period of the wave,
T=0.001 s
The distance to be travelled,
s=4 km = 4000 m.
We know that, speed of sound, v=λT
Thus, v=0.2 m0.001 s
⇒v=200 m s−1
The time taken by the wave to travel a distance of 4000 m,
t=DistanceSpeed
⇒t=4000 m200 ms−1=20 s