A sailor strikes the side of his ship just below the waterline. He hears the echo of the sound reflected from the ocean floor directly below 3.5 s later. How deep is the ocean at this point?

2625 m deep

Explanation:

Let the sound speed in sea water be 1500 m/s. If he hears the echo 3.5s after the strike, then the sound would have traveled a distance of 1500 * 3.5 = 5250 m to the bottom and back. This would mean the ocean is 5250 / 2 = 2625 m deep.

it has to be b, about 97 percent.