18. over time, how do sand dunes tend to migrate? a. perpendicular to the movement of the wind b. in the same direction as the wind blows c. toward the wind d. in random directions

in the same direction as the wind blows

Explanation:

The dunes are hills formed by grains of sand held together by the action of wind or water. Over time, as sand dunes tend to migrate in the same direction as the wind blows.

They have a larger side that is eroded (windward) and a smaller, steeper side (leeward) where deposition of eroded material occurs. Through this mechanism of erosion and deposition the dunes are considered highly dynamic, "moving" over time.