Fact 1: Contact between rocks over long periods of time causes rocks to smooth.

Fact 2: Type of Rocks Salt, quartz and basalt are decent types of aquarium rocks.

Given the two facts above, answer the question "what needs contact to smooth over long periods of time?" with the following options: 
- oxygen
 - soil
 - an object
 - h2o
 - rivers
 - quartz
 - cartilage
 - bamboo


 

      quartz   