Sand dunes are large piles sand that can be found in the desert. what causes these sand dunes?
a.)water
b.)wind
c.)earthquakes
d.) volcanoes

The answer is B of course the wind makes the sand pile up

B the wind takes the sand and blows it to several areas and causes build up in that spot which makes the sand dunes in the desert

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.