challenge: What was the criteria for mobilizing warriors for the Battle of Ōshū, as described in Minamoto no Yoritomo's letter from February 9, 1189?
  
  A. Only warriors from the Ise and Aki Province
  B. Those with a history of loyalty to the Minamoto clan
  C. Those 'worthy of arms'
  D. All able-bodied men in the country
  Answer: 