Fact 1: Chemical weathering causes iron in rock to oxidize.

Fact 2: Rust colored residues are due to oxidized iron.

Given the two facts above, answer the question "What causes rust?" with the following options: 
- a pathogens
 - Roundworms
 - flow of electrons
 - dust
 - thunder
 - an amoebae
 - chemical weathering
 - skin


 

       chemical weathering  