The author's argument concerning Notre-Dame's flying buttresses depends on which of the following assumptions about the choir's lower flyers? A. They accurately reproduce the decoration on the choir's original lower flyers. B. They have a type of decoration used exclusively for exterior surfaces. C. They were the models for the choir's original upper flyers. D. They were the models for the nave's original lower flyers. E. They were constructed after the nave's flyers were constructed.