Lungs are

Options

A. Thin-walled and elastic
B. Thick-walled and inelastic
C. Thick-walled and elastic
D. Thin-walled and inelastic.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structure of Lungs:**
- The lungs are essential organs for the respiratory process in animals. They facilitate the exchange of gases (oxygen and carbon dioxide) between the air and the bloodstream.

2. **Identifying the Membrane:**
- The lungs are lined by a double membrane known as the pleura. This pleural membrane consists of two layers:
- **Visceral Pleura:** The inner layer that is directly attached to the lungs.
- **Parietal Pleura:** The outer layer that is attached to the chest wall.

3. **Pleural Cavity:**
- Between the visceral and parietal pleura, there is a space called the pleural cavity. This cavity contains pleural fluid, which helps reduce friction between the lung surfaces during breathing movements.

4. **Wall Characteristics of Lungs:**
- The membranes lining the lungs are thin, which allows for efficient gas exchange. Therefore, the lungs can be classified as having thin walls.

5. **Elasticity of Lungs:**
- The lungs are also elastic structures. This elasticity is crucial for their function, as it allows the lungs to expand during inhalation (inspiration) and contract during exhalation (expiration).

6. **Conclusion:**
- Based on the characteristics discussed, we conclude that the lungs are both thin-walled and elastic. Thus, the correct answer to the question "Lungs are" is that they are thin-walled and elastic structures.

**Final Answer:**
- Lungs are thin-walled and elastic.

---