challenge: Who were some of the guest performers at Seo's comeback concert 'Christmalowin' in support of the album Quiet Night?
  
  A. IU, Ellie, Swings, and Vasco
  B. Ellie, Swings, and Red Velvet
  C. IU, Swings, and Blackpink
  D. IU, Ellie, and BTS
  Answer: 