The Following Argument Is Valid or Forceful

The following argument is valid or forceful. Identify the irrelevant premise: If General Custer never really made a "last stand" at the Battle of the Little Bighorn, then the "Custer myth" is actually a myth. In fact, this is correct, since the indians basically swept over Custer

A) The indian forces swept over the 7th Cavalry like a tsunami.
B) General Custer was in command of the 7th Cavalry.
C) Sitting Bull correctly prophesied that the indians would win.