You have prepared actin filament seeds in a microfluidic chamber through which you then pass either unlabeled or fluorescently labeled actin subunits in alternation several times, keeping the total monomer concentration constant. You then observe the resulting filaments under a fluorescence microscope. If the filaments appear as drawn schematically in the example below (in which fluorescence is indicated by black color), which end do you think is the plus end-(a) or (b)? Was the concentration of free actin subunits below or above the critical concentration (Cc) for polymerization at the barbed end?