Which substance combines with iron in the presence of water to form rust?
A: acid
B: oxygen
C:nitrogen
D: carbon dioxide
B: oxygen
The substance that combines with iron in the presence of water to form rust is B: oxygen.
The substance that combines with iron in the presence of water to form rust is oxygen, which is option B.
Rust, also known as iron oxide, forms when iron reacts with oxygen and water. This reaction is called oxidation. The presence of water is crucial for the process to happen. Oxygen from the air combines with iron on the surface of the metal, creating iron oxide, which is the reddish-brown substance known as rust. So, in this case, the correct answer is option B: oxygen.