Which of these statements is correct about alveoli?

Options

A. They form a very large surface area.
B. They have a very thin wall.
C. They are covered with blood capillaries.
D. All of these

Explanation

### Step-by-Step Solution:

1. **Understanding Alveoli**: Alveoli are tiny air sacs located in the lungs. Their primary function is to facilitate the exchange of gases (oxygen and carbon dioxide) between the air and the blood.

2. **Evaluating the Statements**:
- **Statement 1**: Alveoli form a very large surface area.
- This statement is correct. The structure of alveoli allows for a large surface area, which is essential for efficient gas exchange.

- **Statement 2**: Alveoli have a very thin wall.
- This statement is also correct. The walls of the alveoli are thin, which aids in the diffusion of gases.

- **Statement 3**: Alveoli are covered with blood capillaries.
- This statement is correct as well. Alveoli are surrounded by a network of capillaries that facilitate the exchange of gases with the blood.

- **Statement 4**: All of these.
- Since all the previous statements are correct, this statement is also correct.

3. **Conclusion**: The correct answer is option 4, "All of these," as all the statements about alveoli are accurate.