The tallest sand dunes in North America are found in Great Sand Dunes National Park and Preserve in Colorado. These dunes were formed from the sand at the bottom of a dry lake bed. Which of the following agents is most likely responsible for the formation of the sand dunes?a) gravityb) icec) waterd) wind

1. crust 1 the metallic layer at the earth's center 2. inner core 3 the firm layer between the crust and the asthenosphere 3. lower mantle 2 the layer surrounding the inner core 4. asthenosphere 4 the semi-solid layer between the top layer and the upper mantle 5. upper mantle 5 the layer between the asthenosphere and lower mantle 6. top layer 6 the earth's outermost layer 7. outer core 7 the hottest part of the mantle