You are presented with the question "what needs contact to smooth over long periods of time?" and the following answer choices: 
- oxygen
 - soil
 - an object
 - h2o
 - rivers
 - quartz
 - cartilage
 - bamboo

Now knowing that contact between rocks over long periods of time causes rocks to smooth and type of Rocks Salt, quartz and basalt are decent types of aquarium rocks, choose the best answer.


 

      quartz   