**Core Concept:** Gas diffusion is a passive transport process that occurs in biological membranes, particularly in the alveoli of the lungs, where oxygen (O2) and carbon dioxide (CO2) diffuse across the membrane based on their concentration gradient. The process relies on the lipid bilayer's hydrophobic nature to prevent direct contact between the gas molecules and the aqueous environment.

**Why the Correct Answer is Right:** Less dense refers to the relative difference in molecular weights between O2 and CO2. Oxygen has a molecular weight of approximately 32 g/mol, while carbon dioxide has a molecular weight of around 44 g/mol. This difference in molecular weight results in a lower density for O2, making it easier for it to diffuse across the alveolar membrane.

**Why Each Wrong Option is Incorrect:**

A. **Less dense does not explain the difference in diffusion rates**: The correct explanation is based on the molecular weights of O2 and CO2, not their density.

B. **Less dense is not relevant to passive transport**: Diffusion is a passive transport process that occurs due to the concentration gradient and molecular weights, not density.

C. **Less dense does not describe the difference in diffusion rates**: As explained above, the molecular weight difference is the key factor in the diffusion rates of O2 and CO2.

D. **Less dense does not address the passive transport mechanism**: The correct explanation is based on the concentration gradient and molecular weights, not density.

**Clinical Pearl:** Understanding the factors affecting gas diffusion, such as molecular weight and concentration gradient, is crucial for understanding respiratory physiology and pathophysiology. This knowledge is essential for interpreting clinical scenarios involving respiratory compromise, like hypoxia and hypercapnia, and their management strategies.