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 gave my students this multiple choice question: Which is an example of substances reacting chemically?

Only one answer is correct among these 4 choices:
- iron and oxygen forming rust
- paper being cut into small pieces
- chocolate melting in a pan
- flour and sugar mixed together in a bowl

Could you tell me which one is correct?

iron and oxygen forming rust