Once Pan had the audacity to compare his music with that of Apollo, and to challenge Apollo, the god of the kithara, to a trial of skill. Tmolus, the mountain-god, was chosen to umpire. Pan blew on his pipes, and with his rustic melody gave great satisfaction to himself and his faithful follower, Midas, who happened to be present. Then Apollo struck the strings of his lyre. Tmolus at once awarded the victory to Apollo, and all but Midas agreed with the judgment. He dissented and questioned the justice of the award. Apollo would not suffer such a depraved pair of ears any longer, and caused them to become the ears of a donkey.

Answer this question, if possible (if impossible, reply "unanswerable"): Who was chosen as umpire of the trial of skill?
Tmolus