Q: 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, what needs contact to smooth over long periods of time?

Available choices:
(A). oxygen.
(B). soil.
(C). an object.
(D). h2o.
(E). rivers.
(F). quartz.
(G). cartilage.
(H). bamboo.
The answer is:
(F).