challenge: Which of the following is true about the isomers of triazine?
  
  A. 1,2,4-triazine has nitrogen atoms at positions 1, 2, and 4
  B. 1,2,5-triazine has nitrogen atoms at positions 1, 2, and 5
  C. 1,3,5-triazine has nitrogen atoms at positions 1, 3, and 5
  D. 1,2,3-triazine has nitrogen atoms at positions 1, 2, and 3
  Answer: 