A sound wave traveling downward with a speed of about 4000 m per 2nd suddenly slows to 1500 m per 2nd not far below the Earth's surface what has the sound wave most likely encountered

Options:

a) A different kind of rock

b) An underground lake

c) An open cavity

d) A pile of dinosaur bones

Option B. An underground lake

Explanation:

The speed of sound in water ranges from 1450 m/s to 1500 m/s. The speed of the sound wave changes because it has crosses two media of different densities. This wave is bound to have a change in direction because of change in the density and speed of the media it passes.

The 1500 m/s speed in the second medium shows that the medium is a pool of water. The only option that applies is an underground lake

im not to sure what it is

it's a b and