单选题 .  ①The border decoration in medieval manuscripts referred to as pen flourishing reached great artistic heights in the northern Netherlands in the 1400s. ②The regional variants in form that evolved make flourishing a useful tool for localizing and roughly dating manuscripts. ③When the first printed books appeared in that region, many copies were still traditionally decorated by hand with such flourishing. ④Since books' publication can usually be dated with far more accuracy than manuscripts, studying these decorations in early printed books might lead to a more precise dating of the penwork in manuscripts. ⑤It is of less help in localizing the flourishing. ⑥Upon completion, copies of printed books were often sold unadorned, to be decorated elsewhere as commissioned by their buyers.13.  Which of the following best describes the function of the highlighted sentence? ______