flow as currents through liquids form waves that travel through space become less dense and rise collide with other molecules
I gave my students this multiple choice question: Heat transfer by conduction occurs when molecules

Only one answer is correct among these 4 choices:
- flow as currents through liquids
- form waves that travel through space
- become less dense and rise
- collide with other molecules

Could you tell me which one is correct?

collide with other molecules