Answer the following question given this paragraph: 

Since we can observe wind forming sand dunes with these patterns now, we have a good explanation for how the Navajo sandstone formed. The Navajo sandstone is a rock formed from ancient sand dunes. The rock shows that the ancient wind direction changed from time to time.


Q: What forms the changing shapes of sand dunes?

 Choices:

- temperature

- humidity

- magnetism

- wind

A:
wind
