You are presented with the quiz "what between objects formed deep within the Earth causes them to smooth" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "contact between rocks over long periods of time causes rocks to smooth" and "metamorphic rocks are rocks formed by the alteration of preexisting rocks deep within the Earth". 

So, what's the best answer to the question?


 

    contact       