Answer the following question given this paragraph: 

Once a sand dune forms, it may slowly migrate over the land. The wind moves grains of sand up the gently sloping side of the dune. This is done by saltation. When the sand grains reach the top of the dune, they slip down the steeper side. The grains are pulled by gravity. The constant movement of sand up and over the dune causes the dune to move along the ground. It always moves in the same direction that the wind usually blows. Can you explain why?.


Q: The migration of what familiar coastal landform involves saltation and gravity?

 Choices:

- volcanos

- lakes

- rivers

- sand dunes

A:
sand dunes
