rust forming on an iron nail sugar crystals dissolving in water sodium and chlorine forming table salt hydrogen and oxygen reacting to produce water
I gave my students this multiple choice question: Which of the following is an example of the formation of a mixture?

Only one answer is correct among these 4 choices:
- rust forming on an iron nail
- sugar crystals dissolving in water
- sodium and chlorine forming table salt
- hydrogen and oxygen reacting to produce water

Could you tell me which one is correct?

sugar crystals dissolving in water