The diffusion membrane is made up of:
(A) Thick columnar epithelium of alveoli
(B) Endothelium of alveolar capillaries
(C) Basement substances in between capillaries and alveoli

Answer

To solve the question regarding the composition of the diffusion membrane, we will analyze each option provided:

### Step-by-Step Solution:

1. **Understanding the Diffusion Membrane**: The diffusion membrane is a structure that facilitates the exchange of gases (oxygen and carbon dioxide) between the alveoli in the lungs and the blood in the capillaries.

2. **Analyzing Option (A)**:
- **Thick columnar epithelium of alveoli**:
- The alveoli are lined with a type of epithelium that is not thick columnar. Instead, they are lined with simple squamous epithelium, which is thin and allows for efficient gas exchange.
- Therefore, this option is **incorrect**.

3. **Analyzing Option (B)**:
- **Endothelium of alveolar capillaries**:
- The capillaries surrounding the alveoli are lined with endothelial cells, which are also simple squamous cells. This thin layer facilitates the diffusion of gases.
- Therefore, this option is **correct**.

4. **Analyzing Option (C)**:
- **Basement substances in between capillaries and alveoli**:
- There is a basement membrane that lies between the alveolar epithelium and the endothelium of the capillaries. This membrane is part of the diffusion barrier and plays a role in gas exchange.
- Therefore, this option is **correct**.

5. **Conclusion**:
- The correct options are (B) Endothelium of alveolar capillaries and (C) Basement substances in between capillaries and alveoli.
- Option (A) is incorrect as the epithelium in the alveoli is not thick columnar but rather simple squamous.

### Final Answer:
The diffusion membrane is made up of:
- (B) Endothelium of alveolar capillaries
- (C) Basement substances in between capillaries and alveoli