You are presented with the question "What is erosion?" and the following answer choices: 
- moonlight
 - looseness of dirt or compost
 - permineralization
 - water vapor
 - fog
 - The area was once underwater
 - fissures of hot, acidic water
 - contact between rocks

Now knowing that contact between rocks over long periods of time causes rocks to smooth and smoothed, grooved and polished rocks are produced by glacial erosion, choose the best answer.


 

        contact between rocks 