Question

A sound wave traveling downward with a speed of about 4,000 m/s suddenly slows to 1,500 m/s not far below the Earth’s surface.

Answer 1

Speed of sound wave depends on the elasticity of medium

It is given by formula

$v = \sqrt{\frac{E}{\rho}}$

now if we are given that initially sound wave is travelling with speed 4000 m/s

Then its speed becomes 1500 m/s

so here since speed is decreased by large value

so here elasticity must decreased by large amount

A. a different kind of rock

B. an underground lake

C. an open cavity

D. a pile of dinosaur bones

So in above all options the density is changed or decreased by large value if the medium is changed to solid to liquid

So the correct answer will be

B. an underground lake

Answer 2

Answer: An underground lake

Hope that helped :D