Fact 1: Sand dunes are formed by the wind transporting and depositing sand.

Fact 2: Winds are caused by pressure differences.

Given the two facts above, answer the question "Sand dunes can be formed by" with 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.    