challenge: What event prompts Goong-dahl to receive an invitation to a duel on the rooftop after school?
  
  A. He threatens a group of old students who were bullying Min-ah
  B. He accidentally breaks a school rule and is challenged by Jae-koo
  C. He beats Jae-koo in a game of basketball
  D. He rescues Min-ah from a group of old teachers
  Answer: 