Which statement describes the function of the freshwater layer?

• A. It increased the salinity of the water.
• B. It cooled the water temperature.
• C. It provided extra oxygen.
• D. It was hypothesized to act as a barrier preventing sea stars from climbing.

Answer: (D)

The essential idea is that a freshwater layer sitting on top of saltwater creates a boundary that can block movement of organisms that live in seawater. Freshwater is less dense than seawater, so it stays on the surface and reduces mixing between the layers. Sea stars are adapted to higher-salinity conditions, and crossing into the fresher layer would disrupt their osmoregulation, making movement through that boundary energetically costly or even harmful. So the layer functions as a barrier that can prevent sea stars from climbing into or through it.

That’s why the statement describing it as a barrier is the best choice. The other options don’t fit because the freshwater layer doesn’t increase salinity; it lowers it. It isn’t primarily about cooling the water, and it doesn’t inherently provide extra oxygen.