Which of these terms, Diffusion Coefficient, Steady$-$state Diffusion, Non Steady$-$state Diffusion, Diffusion Flux best describes the following sentences?

a$)$ The quantity of mass diffusing perpendicular to a unit cross$-$sectional area of material per unit time

b$)$ The constant of proportionality that represents the rate of atomic diffusion

c$)$ The diffusion condition for which there is no net accumulation or depletion of diffusing species

d$)$ The diffusion condition for which there is some net accumulation or depletion of diffusing species