What Role Was Played by Fossils of the One-Toed Horse

What role was played by fossils of the one-toed horse when paleoanthropologists tried to assign firm dates to hominid-fossil-bearing layers at the eastern African site of Koobi Fora?

A) The presence the fossils of one-toed horses confirmed that the horses appeared at Koobi Fora over half a million years earlier than they appeared in the Lower Omo Valley, a nearby site.
B) The presence of the fossils at Koobi Fora, but not at Omo, convinced paleoanthropologists that previous biostratigraphy at the two sites was mistaken.
C) The discrepancy between the one-toed horse fossil dates and the potassium-argon date caused paleoanthropologists to reexamine the potassium-argon date for Koobi Fora.
D) The discrepancy between the one-toed horse fossil dates and the potassium-argon date caused paleaoanthropologists to realize that the hominid fossils at Koobi Fora were older than the Omo fossils.