When wind drops its load of sand, it can form a hill called

Correct Sand dune

Sand dunes are formed when waves transport the sand onto the beach, it is formed on the beach or in the desert. It grows as grains of sand accumulate and form sand dunes.

For the formation of a sand dune, it is necessary to have a large amount of loose sand with less or no vegetation, and a wind breeze is required which will blow the sand. Finally,  an obstacle is needed that will stop the momentum of loose sand and help to settle down and form a sand dune.  

the answer is b)3/8x+1

he is very short, and he can only reach halfway up on the elevator buttons. but on a rainy day, he has his umbrella with him, and he uses it to poke the button at the top.