Ciliated Gills in Clams, Sharks Continuously Swimming with Their Mouths $\Delta$

Ciliated gills in clams, sharks continuously swimming with their mouths partly open, countercurrent flow of blood and water in fish gills, and the use of air sacs to effect unidirectional flow of air through bird lungs are all adaptations that optimize what factor in the formula for Fick's Law of diffusion?

A) D
B) A
C) $\Delta$ p
D) d