The author is arguing for a new digital publishing model that will be lean and cost-effective, eliminating many of the overhead costs associated with traditional publishing. This model will lead to authors requesting higher royalty payment that are more likely to be accommodated first by new digital-publishing firms.
(C) tells us that the demands of higher royalties would be the impetus for completing the transition. Lines 44-51 allow us to make this inference - as the opportunity to bid for new manuscripts will go first to upstart-digital publishing firms, to remain competitive, traditional publishers will have to reduce their redundant functions, i.e. switch to the new digital model.
(B) tells us that the ease of keeping books "in print" will be the primary factor in the eventual acceptance of the new model. The author argues that "given the economic efficiency and convenience for customers," the new model is likely to eventually supplant ora t least rival traditional publishing - though it will be some time before a catalog of printable digital books becomes large enough (lines 22-27). This sentence suggests that convenience and economic efficiency will be the primary factors, and keeping books "in print" might be a more distant goal given the time needed to build a more extensive catalog of printable books.
