What was the significance of the layer of fresh water in Garza and Robles's experiment?

• A. It accelerated sea stars' climbing.
• B. It had no effect on climbing.
• C. It dissolved quickly and caused error.
• D. It was hypothesized to act as a barrier preventing sea stars from climbing.

Answer: (D)

The main idea this item tests is how an environmental boundary can influence an animal’s movement. The freshwater layer was introduced as a potential barrier to crossing. Since sea stars are marine life, a layer of freshwater would present an osmotic and habitat change that they’re likely to avoid, so the layer’s significance is its role as a barrier designed to prevent sea stars from climbing past it. If the layer functions as a barrier, it supports the idea that changes in salinity or exposure to fresh water can impede movement. The other possibilities don’t fit with the experimental goal: the setup wasn’t about accelerating climbing, or about the barrier dissolving or causing errors, but about whether the barrier would hinder climbing in the first place.