iron and oxygen forming rust paper being cut into small pieces chocolate melting in a pan flour and sugar mixed together in a bowl
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: Which is an example of substances reacting chemically?
Possibilities:
- iron and oxygen forming rust
- paper being cut into small pieces
- chocolate melting in a pan
- flour and sugar mixed together in a bowl

iron and oxygen forming rust