Although the earliest surviving Greek inscriptions written in an alphabet date from the eighth century B.C., a strong case can be made that the Greeks actually adopted alphabetic writing at least two centuries earlier. Significantly, the text of these earliest surviving Greek inscriptions sometimes runs from right to left and sometimes from left to right. Now, the Greeks learned alphabetic writing from the Phoenicians, and in the process they would surely have adopted whatever convention the Phoenicians were then using with respect to the direction of writing. Originally, Phoenicians writing ran in either direction, but by the eighth century B.C. it had been consistently written from right to left for about two centuries.

In the argument given, the two portions in boldface play which of the following roles?

【选项】The first is an assumption that the argument concludes is unjustified; the second presents part of the grounds for that conclusion.