Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Look at this rusty bike. It has been left outside in damp weather too many times, so the iron in the metal parts has rusted. Iron rusts when it combines with oxygen in the air. Iron rusting is an example of a chemical reaction. In a chemical reaction, substances change into entirely different substances. For example, the iron in the bike and the oxygen in the air have changed into rust. Answer: rusted
A rusty bike has been left outside in damp weather too many times, so the iron in the metal parts have?