yarn being knitted into a sweater the engine from a tractor being started the metal on a bicycle forming rust grass capturing the energy from sunlight
I gave my students this multiple choice question: Which of these best describes a change that is physical?

Only one answer is correct among these 4 choices:
- yarn being knitted into a sweater
- the engine from a tractor being started
- the metal on a bicycle forming rust
- grass capturing the energy from sunlight

Could you tell me which one is correct?

yarn being knitted into a sweater