In the context of the passage as a whole, the primary function of the highlighted sentence is to A. emphasize the influence of a particular social class on the conventions of eighteenth-century English portraiture B. account for the origin of a particular type of behavior frequently represented in eighteenth-century English portraiture C. acknowledge a historical basis for two competing hypotheses about a particular portrait type D. question the relevance of certain evidence frequently cited in support of an explanation for a particular portrait type E. concede that one explanation for the prevalence of a particular portrait type has a basis in fact