**Explanation:**

The correct answer is option 'D': The CO2 pressure gradient is larger.

**Diffusion of Gases Across the Alveolar Membrane:**

The alveolar membrane is a thin, semipermeable membrane that separates the alveoli in the lungs from the capillaries. It allows for the exchange of gases between the air in the alveoli and the blood in the capillaries.

During respiration, oxygen (O2) and carbon dioxide (CO2) need to cross the alveolar membrane in order to be exchanged between the lungs and the blood. The diffusion of these gases is influenced by several factors, including solubility, concentration gradients, and membrane thickness.

**Factors Affecting Diffusion of Gases:**

1. **Solubility:** The solubility of a gas in a liquid determines how readily it can dissolve and pass through a membrane. CO2 is more soluble in water (H2O) than O2, which enables it to pass the alveolar membrane more readily. However, this is not the main reason why CO2 diffuses faster than O2.

2. **Concentration Gradient:** The concentration gradient of a gas across a membrane is the difference in its partial pressure on either side of the membrane. The greater the difference in partial pressure, the faster the diffusion. In the case of CO2 and O2, the concentration gradient of CO2 is much larger than that of O2, primarily due to differences in their production and removal rates in the body.

3. **Membrane Thickness:** The thickness of the alveolar membrane affects the rate of diffusion. A thinner membrane allows for faster diffusion. However, the alveolar membrane is already very thin, so the difference in thickness between CO2 and O2 diffusion is negligible.

**Why is the CO2 Pressure Gradient Larger?**

The main reason why the diffusion of CO2 across the alveolar membrane is 20 times faster than that of O2 is because the pressure gradient for CO2 is larger. This means that there is a greater difference in the partial pressure of CO2 between the alveoli and the capillaries, driving a more significant diffusion.

CO2 is produced as a waste product of cellular metabolism and is continuously generated in the tissues. It diffuses into the capillaries, resulting in a higher partial pressure of CO2 in the blood compared to the alveoli. This large pressure gradient facilitates the rapid diffusion of CO2 from the blood into the alveoli.

On the other hand, O2 is taken up by the blood in the capillaries and transported to the tissues for cellular respiration. The partial pressure of O2 in the alveoli is higher than that in the capillaries, but the pressure gradient is not as significant as that of CO2. This is because O2 is continuously being consumed by the tissues, reducing the partial pressure difference between the alveoli and the capillaries.

Overall, the larger CO2 pressure gradient, driven by its continuous production and removal in the body, is the primary reason for its faster diffusion across the alveolar membrane compared to O2.