You are presented with the question "Sand dunes can be formed by" and the following answer choices: 
- air mass over Earth's surface
 - movements in the earth's crust
 - The bending of a ray of light
 - Movement of an air mass
 - pressure differences.
 - Physical weathering
 - looseness of dirt or compost
 - remains of prehistoric life

Now knowing that sand dunes are formed by the wind transporting and depositing sand and winds are caused by pressure differences, choose the best answer.


 

     pressure differences.    