Choices:
(a). air mass over Earth's surface
(b). movements in the earth's crust
(c). The bending of a ray of light
(d). Movement of an air mass
(e). pressure differences.
(f). Physical weathering
(g). looseness of dirt or compost
(h). remains of prehistoric life
Given those answer options, answer the question: You are presented with the quiz "Sand dunes can be formed by" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "sand dunes are formed by the wind transporting and depositing sand" and "winds are caused by pressure differences". So, what's the best answer to the question?
A:
(e).