A. Chronological order
When dating artifacts, sometimes it is impossible to directly place a date on an item in question. However, sometimes it is possible to know the relation of the artifact in question to other artifacts whose date of creation or existence is known. And, because of this, it is possible to determine an approximate age, not by directly dating, but by relative dating. Thus, if you know the artifact in question existed after Artifact A, but before Artifact B, because of the chronology of things, you’d be able to come up with an approximation.