a flag on a pole the temperature on a thermometer birds flying above the trees the shadow of a building
I gave my students this multiple choice question: Ruben is using a compass to find out which direction the wind is blowing. Which of the following would be best for him to observe to help him find the direction of the wind?

Only one answer is correct among these 4 choices:
- a flag on a pole
- the temperature on a thermometer
- birds flying above the trees
- the shadow of a building

Could you tell me which one is correct?

a flag on a pole