You are presented with the question "What can friction due to gravity cause?" and the following answer choices: 
- smoothing
 - energy
 - heat energy
 - waves
 - erosion
 - sweating
 - an object
 - Energy.

Now knowing that contact between rocks over long periods of time causes rocks to smooth and gravity can exert forces that cause friction in rocks and ice, choose the best answer.


 

 smoothing        