Diffusion efficiency limited by path length in block dimensions

Steps:

• Diffusion follows Fick's law: rate inversely proportional to distance, time proportional to distance squared.
• Efficiency measures how quickly substances reach block interiors from surfaces.
• For fixed volume, elongation stretches one dimension, increasing maximum path length to farthest interior points.
• Longer paths slow overall diffusion, reducing efficiency compared to compact shapes.

Why C is correct:

• Elongation increases the longest dimension, extending diffusion distance and thus time per the diffusion equation (time ∝ distance²), decreasing efficiency.

Why the others are wrong:

• A: Width increase raises volume without fixed constraints, so surface-to-volume ratio change is unclear; no guaranteed decrease.
• B: Flattening height shortens path in that direction, increasing efficiency, opposite of stated.
• D: Larger size decreases surface-to-volume ratio, reducing efficiency, not increasing.

Final answer: C