The plasma membrane is selective because it has to be able to determine what is vital to the body or what is dangerous and unable to be received. Specifically the plasma membrane is a part of the cell that is contained of lipids and proteins. It is the regulation center of the cell and it's operation is functional to a living organism.
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A - Salinity because salinity is how much salt is in water and adding more would increase the salinity.
A sea sponge is what mimics a synthetic sponge