Answer: 0 represents the surface of the ocean
Step-by-step explanation: Words like 'below' indicate a negative number. Since the dolfin's positions is -9 below the sea level, the equation x-9=-9 can be created (let x = ocean surface level). Add 9 to both sides to cancel out -9 on the left side and isolate x. You have x = 0, so the ocean surface is 0.
Of course, you don't have to do all that! That was a in-depth explanation. Just make sure to look at the wording :P
