Consider the dissolution of a sparingly soluble solid wall in a falling liquid film in fully-developed laminar flow. Which of the following assumptions could justify neglecting convective transport in the transverse direction (i.e., normal to the wall) and why?

(a) The mass transfer rate is sufficiently small (b) The diffusion coefficient is sufficiently large (c) The transverse concentration gradient is negligible (d) The velocity profile near the wall is approximately linear