a student passes through a door two students pass by each other two students bump into each other students pass from a crowded room into a hall
I gave my students this multiple choice question: Which of these would be a model of diffusion?

Only one answer is correct among these 4 choices:
- a student passes through a door
- two students pass by each other
- two students bump into each other
- students pass from a crowded room into a hall

Could you tell me which one is correct?

students pass from a crowded room into a hall