A recently unearthed sculpture appears to be the work of either the ancient Greek sculptor Achermus, who lived in Chios, or Leochares of Athens, whose sculptures sometimes resembled those of Achermus. Analysis of the sculpture reveals that it is made of a marble quarried near Chios; the marble found near Athens is a different color. Thus, the sculpture was probably made by Achermus.
Which of the following is an assumption that the argument requires?