Pioneer species, I think.
Answer:
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. <u>These functions are given by the specific characteristic of the epithelium considering its location.</u> 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, <u>the fluids can diffuse easily than if the epithelium had many layers or cubical cells.</u>
Involuntary action in body