You are presented with the question "what smooths over long periods of time due to contact?" and the following answer choices: 
- oxygen
 - calcite
 - h2o
 - clouds
 - erosion
 - shale
 - bamboo
 - rivers

Now knowing that contact between rocks over long periods of time causes rocks to smooth and primary rock types are Sandstone, and Shale, choose the best answer.


 

      shale   