challenge: During which emperors' reigns did the position of viceroy exist in Vietnamese history?
  
  A. Minh Mạng Emperor and Thiệu Trị Emperor
  B. Thiệu Trị Emperor and Tự Đức Emperor
  C. Minh Mạng Emperor and Tự Đức Emperor
  D. Gia Long Emperor and Minh Mạng Emperor
  Answer: 