You are presented with the question "objects will sink between boundaries with different what due to gravity" and the following answer choices: 
- Energy.
 - surfaces
 - densities
 - forces
 - rocks
 - minerals
 - energy
 - heat energy

Now knowing that if an object in a fluid is more dense than that fluid then that object will sink in that fluid because of gravity and waves can be created at the boundary between fluids of different densities, choose the best answer.


 

   densities      