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

Fact 2: Metamorphic rocks are rocks formed by the alteration of preexisting rocks deep within the Earth.

Given the two facts above, answer the question "what between objects formed deep within the Earth causes them to smooth" with the following options: 
- Movement
 - loose soil
 - erosion
 - contact
 - soils
 - break off
 - friction
 - Bend


 

    contact     