Although the earliest records we have of Arabic sources using the numeral zero date back to 773 C.E., the fact that the Arabic civilization had been performing advanced mathematical operations for a century before that indicates that they understood the concept of numerical zero long before they ever put it in writing. After all, Indians and Babylonians had both developed the concept independently before then, and the Arabic people could reasonably be expected to have acquired similar concepts. And although there is a dearth of writing indicating the exchange, by the 6th century C.E. Arabs had been in close contact with Indian civilization for a prolonged period.
In the argument given, the two portions in boldface play which of the following roles?