a$.$ Differentiate between mass$-$transfer coefficients for equimolar counter$-$diffusion and diffusion of A through stagnant, non$-$diffusing B$.(4$ marks$)$ b$.$ Describe the three regions of mass transfer that can be visualized in a turbulent flow system and explain the dominant mechanisms of mass transfer within each region. $(6$ marks$)$ c$.$ Describe the concept of permeability in relation to diffusion in solids. Explain the relationship between permeability, diffusivity, and solubility.