Which of the following ____________ is not true regarding the unsteady state diffusion through a semi-infinite slab? (a) Diffusive flux of the slab varies with both time and position; (b) Diffusive flux at the interface (z=0) is a constant; (c) Concentrations at different positions of the slab vary with time; (d) Concentration at different positions of the slab can be obtained by solving the partial differential equation analytically.