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

Fact 2: Rocks are actually solid mineral deposits.

Given the two facts above, answer the question "Contact between solid mineral deposits over long periods of what causes them to smooth" with the following options: 
- time
 - vacuum
 - widen
 - cracks
 - clocks
 - years
 - Bend
 - instant


 

 time        