___ is composed of just one cell layer.

Options

A. Capillary
B. Arteriole
C. Venule
D. Vein

Answer

To answer the question "___ is composed of just one cell layer," we need to identify the type of blood vessel that has a wall made up of a single layer of cells.

### Step-by-Step Solution:

1. **Understanding Blood Vessel Structure**: Blood vessels are categorized into arteries, arterioles, capillaries, venules, and veins. Each of these has a different structure and function.

2. **Identifying Capillaries**: Among these blood vessels, capillaries are the smallest and have a unique structure. They are designed to facilitate the exchange of substances between the blood and surrounding tissues.

3. **Cell Layer Composition**: Capillaries are composed of a single layer of endothelial cells. This thin structure allows for efficient diffusion of gases and nutrients.

4. **Comparison with Other Blood Vessels**: In contrast, arteries, arterioles, venules, and veins have more complex structures with multiple layers, including smooth muscle and connective tissue, which makes them thicker than capillaries.

5. **Conclusion**: Therefore, the correct answer to the question is "capillary," as it is the only type of blood vessel that is composed of just one cell layer.

### Final Answer:
**Capillary**