Diffusion across the alveolar wall Steps:

• Molecule starts in alveolar space directly adjacent to the outer cell membrane (distance 0 nm to membrane).
• Diffuses across the human cell membrane: 5 nm.
• Diffuses through the cytoplasm of alveolar wall cells: 90 nm.
• Reaches the inner surface of the alveolar wall, adjacent to the capillary wall touched by the RBC (total distance 95 nm).

Why C is correct:

• The path requires crossing only the alveolar wall's membrane and cytoplasm to reach the RBC, per the given thicknesses and assumption of contact.

Why the others are wrong:

• A: Adds 300 nm tissues to 95 nm (295 nm too low; likely misadds to reach 595 nm).
• B: Rough estimate ignoring precise membrane + cytoplasm sum.
• D: Adds 300 nm tissues plus extra 15 nm (two membranes), inflating the path.

Final answer: C