Doubling diffusion rate by altering key factors

Steps:

• Recall the diffusion rate formula: rate ∝ (surface area × concentration difference) / membrane thickness.
• Assume standard changes: 1 doubles surface area (doubles rate); 2 halves concentration difference (halves rate); 3 doubles thickness (halves rate); 4 halves thickness (doubles rate).
• Identify combinations that multiply rate by 2: doubling area (×2) or halving thickness (×2), but not both unless specified.
• Test options: only 1 and 4 each double the rate independently or combined without excess.

Why C is correct:

• Option C (1 and 4) doubles surface area and halves thickness, each factor multiplying rate by 2 for a net doubling per the proportionality formula.

Why the others are wrong:

• A includes 2, which halves concentration difference and thus halves the rate.
• B includes 3, which doubles thickness and halves the rate.
• D includes 2 and 3, both halving the rate for a quarter effect.

Final answer: C