Sand dunes are formed by the wind transporting and depositing sand, and winds are caused by pressure differences. Given these facts, sand dunes can be formed by among the following options:
- 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


 

     pressure differences.    