Skin helps in exchange of gases because

Options

A. It is kept moist by the mucus and water
B. It is thin
C. It is richly supplied with blood
D. All of these

Explanation

**Step-by-Step Solution:**

1. **Understanding the Role of Skin in Gas Exchange:**
- The skin is the outermost layer of many animals and is in direct contact with the environment. This positioning allows for the exchange of gases (like oxygen and carbon dioxide) between the body and the atmosphere.

2. **Moisture of the Skin:**
- The skin is kept moist by mucus and water. A moist surface is crucial for gas exchange because gases dissolve more readily in liquid than in air. Thus, the moisture facilitates the diffusion of gases.

3. **Thickness of the Skin:**
- The skin is relatively thin, which enhances the efficiency of gas exchange. A thinner membrane allows gases to diffuse more easily and quickly, as the rate of diffusion is inversely proportional to the thickness of the membrane.

4. **Rich Blood Supply:**
- The skin is richly supplied with blood vessels. A large blood supply is essential for transporting the exchanged gases (oxygen and carbon dioxide) to and from the tissues. This ensures that the gases can be efficiently delivered to the cells and waste gases can be removed.

5. **Conclusion:**
- All the mentioned factors (moisture, thinness, and rich blood supply) contribute to the skin's ability to facilitate gas exchange. Therefore, the correct answer is that the skin helps in the exchange of gases because it is kept moist, it is thin, and it is richly supplied with blood.

**Final Answer:**
- The correct option is: **All of the above (Option 4)**.

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