Generally, epithelial cell layers are responsible for separating two fluids. For example, the epithelium of blood vessels in animals separates the blood from the interstitial fluid. What characteristic would you expect to see in an epithelium that was specialized for passive diffusion of materials from one fluid to another?

The correct answer is: a single layer of flattened cells.

Explanation:

The epithelium is one of the four basic tissues and has many important functions. These functions are given by the specific characteristic of the epithelium considering its location. For example, an epithelium that specializes in protection (for example, the one found in skin) has lots of layers in order to carry this task.

Epithelium that specializes in the passive diffusion of materials from one fluid to another needs to have a single layer of flattened cells: this way, the fluids can diffuse easily than if the epithelium had many layers or cubical cells.

single layer of flattened cells, with thin walls.

One of the characteristic features  of epithelium is possession of  single layer of flattened  thin cells which are one cell thick. This reduces diffusion distance, and increases the rate of diffusion across  the  walls.

Based on Fick's law, the rate of diffusion is proportional   to concentration gradient, surface area of a medium,but inversely proportional to diffusion distance.

Therefore as the diffusion distance is very short in this epithelium,(flattened ,thin cells)the rate of diffusion will be high across the walls, therefore a good adaptation for the epithelium for passive diffusion.

Explanation:

the correct answer would be 6 feet.