What two pieces of information are needed to determine ocean depth through echo sounding?

Sonar is an instrument which is used to search for things which are underwater. It is also used to find the depth of water by giving out ultrasonic vibrations into the water. The time which the vibrations take to return assists to find the distance of the reflecting surface.  

The correct answer is:

•velocity of the wave and  

•the return time of the wave.

Echo sound is the method for determination the water depth, based on transmitting sound waves into water. Two things that are recorded are the time interval between emission and return of a pulse and the speed of sound in water at the time.

doctors i believe if i'm wrong sorry

b is the best answer for this question.