Fact 1: If an object in a fluid is more dense than that fluid then that object will sink in that fluid because of gravity.

Fact 2: Waves can be created at the boundary between fluids of different densities.

Given the two facts above, answer the question "objects will sink between boundaries with different what due to gravity" with the following options: 
- Energy.
 - surfaces
 - densities
 - forces
 - rocks
 - minerals
 - energy
 - heat energy


 

   densities      