You are presented with the question "what is smoothed by contact over long periods of time?" and the following answer choices: 
- shale
 - earth
 - eras
 - Roads
 - soils
 - water
 - clouds
 - ground

Now knowing that contact between rocks over long periods of time causes rocks to smooth and sandstone, shale, and limestone are sedimentary rocks, choose the best answer.


 

 shale        