Fact 1: Contact between rocks over long periods of time causes rocks to smooth.

Fact 2: Gravity can exert forces that cause friction in rocks and ice.

Given the two facts above, answer the question "What can friction due to gravity cause?" with the following options: 
- smoothing
 - energy
 - heat energy
 - waves
 - erosion
 - sweating
 - an object
 - Energy.


 

 smoothing        