Question

I need help with 21!! Thanksss!!

Answer 1

Enough sand for forming islands, the sea level is rising and there are wind and waves with enough energy to move the sand.

Answer 2

In order for a barrier island to form, the coastal sand is organized into barrier islands. There are three conditions for an Island of this type to form. 
1. There must be a sufficient supply of sand to form the island.
2. The sea level must be rising.
3. There needs to be winds and waves with sufficient energy to move the sand around

Hope that helps